diff --git a/.github/workflows/build.yml b/.github/workflows/build.yml
index 1a78ff02..54ab0852 100644
--- a/.github/workflows/build.yml
+++ b/.github/workflows/build.yml
@@ -2,10 +2,10 @@ name: Tests
 
 on:
   push:
-    branches: [ master ]
+    branches: [ main ]
     paths-ignore: [ README.md ]
   pull_request:
-    branches: [ master ]
+    branches: [ main ]
     paths-ignore: [ README.md ]
   workflow_dispatch:
 
diff --git a/Sources/GraphQL/Error/SyntaxError.swift b/Sources/GraphQL/Error/SyntaxError.swift
index 7f2330a2..26f2eb75 100644
--- a/Sources/GraphQL/Error/SyntaxError.swift
+++ b/Sources/GraphQL/Error/SyntaxError.swift
@@ -50,16 +50,19 @@ func highlightSourceAtLocation(source: Source, location: SourceLocation) -> Stri
 
 func splitLines(string: String) -> [String] {
 
-    let nsstring = NSString(string: string)
-    let regex = try! NSRegularExpression(pattern: "\r\n|[\n\r]", options: [])
-
     var lines: [String] = []
     var location = 0
-
-    for match in regex.matches(in: string, options: [], range: NSRange(0..<nsstring.length)) {
-        let range = NSRange(location..<match.range.location)
-        lines.append(nsstring.substring(with: range))
-        location =  match.range.location + match.range.length
+    
+    let nsstring = NSString(string: string)
+    do {
+        let regex = try NSRegularExpression(pattern: "\r\n|[\n\r]", options: [])
+        for match in regex.matches(in: string, options: [], range: NSRange(0..<nsstring.length)) {
+            let range = NSRange(location..<match.range.location)
+            lines.append(nsstring.substring(with: range))
+            location =  match.range.location + match.range.length
+        }
+    } catch {
+        // Let lines and location remain unchanged
     }
 
     if lines.isEmpty {
diff --git a/Sources/GraphQL/Execution/Execute.swift b/Sources/GraphQL/Execution/Execute.swift
index 486eb994..9d07849e 100644
--- a/Sources/GraphQL/Execution/Execute.swift
+++ b/Sources/GraphQL/Execution/Execute.swift
@@ -670,7 +670,7 @@ public func resolveField(
     let fieldAST = fieldASTs[0]
     let fieldName = fieldAST.name.value
 
-    let fieldDef = getFieldDef(
+    let fieldDef = try getFieldDef(
         schema: exeContext.schema,
         parentType: parentType,
         fieldName: fieldName
@@ -796,7 +796,7 @@ func completeValueCatchingError(
             result: result
         ).flatMapError { error -> EventLoopFuture<Any?> in
             guard let error = error as? GraphQLError else {
-                fatalError()
+                return exeContext.eventLoopGroup.next().makeFailedFuture(error)
             }
             exeContext.append(error: error)
             return exeContext.eventLoopGroup.next().makeSucceededFuture(nil)
@@ -809,7 +809,7 @@ func completeValueCatchingError(
         exeContext.append(error: error)
         return exeContext.eventLoopGroup.next().makeSucceededFuture(nil)
     } catch {
-        fatalError()
+        return exeContext.eventLoopGroup.next().makeFailedFuture(error)
     }
 }
 
@@ -1195,7 +1195,7 @@ func getFieldDef(
     schema: GraphQLSchema,
     parentType: GraphQLObjectType,
     fieldName: String
-) -> GraphQLFieldDefinition {
+) throws -> GraphQLFieldDefinition {
     if fieldName == SchemaMetaFieldDef.name && schema.queryType.name == parentType.name {
         return SchemaMetaFieldDef
     } else if fieldName == TypeMetaFieldDef.name && schema.queryType.name == parentType.name {
@@ -1203,7 +1203,10 @@ func getFieldDef(
     } else if fieldName == TypeNameMetaFieldDef.name {
         return TypeNameMetaFieldDef
     }
-
-    // we know this field exists because we passed validation before execution
-    return parentType.fields[fieldName]!
+    
+    // This field should exist because we passed validation before execution
+    guard let fieldDefinition = parentType.fields[fieldName] else {
+        throw GraphQLError(message: "Expected field definition not found: '\(fieldName)' on '\(parentType.name)'")
+    }
+    return fieldDefinition
 }
diff --git a/Sources/GraphQL/GraphQL.swift b/Sources/GraphQL/GraphQL.swift
index 5cd7cb7e..b4c32870 100644
--- a/Sources/GraphQL/GraphQL.swift
+++ b/Sources/GraphQL/GraphQL.swift
@@ -33,8 +33,11 @@ public struct GraphQLResult : Equatable, Codable, CustomStringConvertible {
     }
     
     public var description: String {
-        let data = try! GraphQLJSONEncoder().encode(self)
-        return String(data: data, encoding: .utf8)!
+        guard let data = try? GraphQLJSONEncoder().encode(self),
+              let dataString = String(data: data, encoding: .utf8) else {
+            return "Unable to encode GraphQLResult"
+        }
+        return dataString
     }
 }
 
diff --git a/Sources/GraphQL/Language/Lexer.swift b/Sources/GraphQL/Language/Lexer.swift
index 0501aa74..f476b26c 100644
--- a/Sources/GraphQL/Language/Lexer.swift
+++ b/Sources/GraphQL/Language/Lexer.swift
@@ -35,8 +35,9 @@ func advanceLexer(lexer: Lexer) throws -> Token {
 
     if token.kind != .eof {
         repeat {
-            token.next = try readToken(lexer: lexer, prev: token)
-            token = token.next!
+            let nextToken = try readToken(lexer: lexer, prev: token)
+            token.next = nextToken
+            token = nextToken
         } while token.kind == .comment
 
         lexer.token = token
@@ -608,7 +609,14 @@ func readString(source: Source, start: Int, line: Int, col: Int, prev: Token) th
         positionIndex = body.utf8.index(after: positionIndex)
 
         if code == 92 { // \
-            value += String(body.utf8[chunkStartIndex..<startIterationIndex])!
+            guard let chunk = String(body.utf8[chunkStartIndex..<startIterationIndex]) else {
+                throw syntaxError(
+                    source: source,
+                    position: body.offset(of: positionIndex),
+                    description: "Unable to initialize String from chunk: \(body.utf8[chunkStartIndex..<startIterationIndex])."
+                )
+            }
+            value += chunk
             currentCode = body.charCode(at: positionIndex)
 
             if let code = currentCode {
@@ -643,8 +651,16 @@ func readString(source: Source, start: Int, line: Int, col: Int, prev: Token) th
                             "\\u\(body.utf8[aIndex...dIndex])."
                         )
                     }
-
-                    value += String(Character(UnicodeScalar(UInt32(charCode))!))
+                    
+                    guard let unicodeScalar = UnicodeScalar(UInt32(charCode)) else {
+                        throw syntaxError(
+                            source: source,
+                            position: body.offset(of: positionIndex),
+                            description:
+                            "Invalid unicode character code: \\u\(charCode)."
+                        )
+                    }
+                    value += String(Character(unicodeScalar))
 
                     positionIndex = dIndex
                 default:
@@ -668,8 +684,15 @@ func readString(source: Source, start: Int, line: Int, col: Int, prev: Token) th
             description: "Unterminated string."
         )
     }
-
-    value += String(body.utf8[chunkStartIndex..<positionIndex])!
+    
+    guard let chunk = String(body.utf8[chunkStartIndex..<positionIndex]) else {
+        throw syntaxError(
+            source: source,
+            position: body.offset(of: positionIndex),
+            description: "Unable to initialize String from chunk: \(body.utf8[chunkStartIndex..<positionIndex])."
+        )
+    }
+    value += chunk
 
     return Token(
         kind: .string,
@@ -702,7 +725,15 @@ func readBlockString(lexer: Lexer, source: Source, start: Int, line: Int, col: I
            body.utf8[body.utf8.index(positionIndex, offsetBy: 1)] == 34,
            body.utf8[body.utf8.index(positionIndex, offsetBy: 2)] == 34 {
             
-            rawValue += String(body.utf8[chunkStartIndex..<positionIndex])!
+            guard let chunk = String(body.utf8[chunkStartIndex..<positionIndex]) else {
+                throw syntaxError(
+                    source: source,
+                    position: body.offset(of: positionIndex),
+                    description: "Unable to initialize String from chunk: \(body.utf8[chunkStartIndex..<positionIndex])."
+                )
+            }
+            rawValue += chunk
+            
             return Token(
                 kind: .blockstring,
                 start: start,
@@ -747,7 +778,15 @@ func readBlockString(lexer: Lexer, source: Source, start: Int, line: Int, col: I
                   body.utf8[body.utf8.index(positionIndex, offsetBy: 2)] == 34,
                   body.utf8[body.utf8.index(positionIndex, offsetBy: 3)] == 34 {
             // escaped triple quote (\""")
-            rawValue += String(body.utf8[chunkStartIndex..<positionIndex])! + "\"\"\""
+            
+            guard let chunk = String(body.utf8[chunkStartIndex..<positionIndex]) else {
+                throw syntaxError(
+                    source: source,
+                    position: body.offset(of: positionIndex),
+                    description: "Unable to initialize String from chunk: \(body.utf8[chunkStartIndex..<positionIndex])."
+                )
+            }
+            rawValue += chunk + "\"\"\""
             positionIndex = body.utf8.index(positionIndex, offsetBy: 4)
             chunkStartIndex = positionIndex
         } else {
diff --git a/Sources/GraphQL/Language/Location.swift b/Sources/GraphQL/Language/Location.swift
index ab5a49fb..1c7275c5 100644
--- a/Sources/GraphQL/Language/Location.swift
+++ b/Sources/GraphQL/Language/Location.swift
@@ -18,13 +18,15 @@ func getLocation(source: Source, position: Int) -> SourceLocation {
     var line = 1
     var column = position + 1
 
-    let regex = try! NSRegularExpression(pattern: "\r\n|[\n\r]", options: [])
-
-    let matches = regex.matches(in: source.body, options: [], range: NSRange(0..<source.body.utf16.count))
-
-    for match in matches where match.range.location < position {
-        line += 1
-        column = position + 1 - (match.range.location + match.range.length)
+    do {
+        let regex = try NSRegularExpression(pattern: "\r\n|[\n\r]", options: [])
+        let matches = regex.matches(in: source.body, options: [], range: NSRange(0..<source.body.utf16.count))
+        for match in matches where match.range.location < position {
+            line += 1
+            column = position + 1 - (match.range.location + match.range.length)
+        }
+    } catch {
+        // Leave line and position unset if regex fails
     }
 
     return SourceLocation(line: line, column: column)
diff --git a/Sources/GraphQL/Language/Parser.swift b/Sources/GraphQL/Language/Parser.swift
index 7c19ed71..3b4f4ba7 100644
--- a/Sources/GraphQL/Language/Parser.swift
+++ b/Sources/GraphQL/Language/Parser.swift
@@ -118,9 +118,12 @@ func parseType(source: Source, noLocation: Bool = false) throws -> Type {
  */
 func parseName(lexer: Lexer) throws -> Name {
     let token = try expect(lexer: lexer, kind: .name)
+    guard let value = token.value else {
+        throw GraphQLError(message: "Expected name token to have value: \(token)")
+    }
     return Name(
         loc: loc(lexer: lexer, startToken: token),
-        value: token.value!
+        value: value
     )
 }
 
@@ -172,8 +175,10 @@ func parseDefinition(lexer: Lexer) throws -> Definition {
     }
     
     if peek(lexer: lexer, kind: .name) {
-        switch lexer.token.value! {
-        // Note: subscription is an experimental non-spec addition.
+        guard let value = lexer.token.value else {
+            throw GraphQLError(message: "Expected name token to have value: \(lexer.token)")
+        }
+        switch value {
         case "query", "mutation", "subscription":
             return try parseOperationDefinition(lexer: lexer);
         case "fragment":
@@ -236,11 +241,13 @@ func parseOperationDefinition(lexer: Lexer) throws -> OperationDefinition {
  */
 func parseOperationType(lexer: Lexer) throws -> OperationType {
     let operationToken = try expect(lexer: lexer, kind: .name)
+    guard let value = operationToken.value else {
+        throw GraphQLError(message: "Expected name token to have value: \(operationToken)")
+    }
 
-    switch operationToken.value! {
+    switch value {
     case "query": return .query
     case "mutation": return .mutation
-    // Note: subscription is an experimental non-spec addition.
     case "subscription": return .subscription
     default: throw unexpected(lexer: lexer, atToken: operationToken)
     }
@@ -458,26 +465,35 @@ func parseValueLiteral(lexer: Lexer, isConst: Bool) throws -> Value {
         return try parseObject(lexer: lexer, isConst: isConst)
     case .int:
         try lexer.advance()
+        guard let value = token.value else {
+            throw GraphQLError(message: "Expected int token to have value: \(token)")
+        }
         return IntValue(
             loc: loc(lexer: lexer, startToken: token),
-            value: token.value!
+            value: value
         )
     case .float:
         try lexer.advance()
+        guard let value = token.value else {
+            throw GraphQLError(message: "Expected float token to have value: \(token)")
+        }
         return FloatValue(
             loc: loc(lexer: lexer, startToken: token),
-            value: token.value!
+            value: value
         )
     case .string, .blockstring:
         return try parseStringLiteral(lexer: lexer, startToken: token)
     case .name:
-        if (token.value == "true" || token.value == "false") {
+        guard let value = token.value else {
+            throw GraphQLError(message: "Expected name token to have value: \(token)")
+        }
+        if (value == "true" || value == "false") {
             try lexer.advance()
             return BooleanValue(
                 loc: loc(lexer: lexer, startToken: token),
-                value: token.value == "true"
+                value: value == "true"
             )
-        } else if token.value == "null" {
+        } else if value == "null" {
             try lexer.advance()
             return NullValue(
                 loc: loc(lexer: lexer, startToken: token)
@@ -486,7 +502,7 @@ func parseValueLiteral(lexer: Lexer, isConst: Bool) throws -> Value {
             try lexer.advance()
             return EnumValue(
                 loc: loc(lexer: lexer, startToken: token),
-                value: token.value!
+                value: value
             )
         }
     case .dollar:
@@ -564,10 +580,13 @@ func parseObjectField(lexer: Lexer, isConst: Bool) throws -> ObjectField {
  */
 
 func parseStringLiteral(lexer: Lexer, startToken: Token) throws -> StringValue {
-    try lexer.advance();
+    try lexer.advance()
+    guard let value = startToken.value else {
+        throw GraphQLError(message: "Expected string literal token to have value: \(startToken)")
+    }
     return StringValue(
         loc: loc(lexer: lexer, startToken: startToken),
-        value: startToken.value!,
+        value: value,
         block: startToken.kind == .blockstring
     )
 }
@@ -668,7 +687,10 @@ func parseTypeSystemDefinition(lexer: Lexer) throws -> TypeSystemDefinition {
         : lexer.token
     
     if keywordToken.kind == .name {
-        switch keywordToken.value! {
+        guard let value = keywordToken.value else {
+            throw GraphQLError(message: "Expected keyword token to have value: \(keywordToken)")
+        }
+        switch value {
         case "schema": return try parseSchemaDefinition(lexer: lexer);
         case "scalar": return try parseScalarTypeDefinition(lexer: lexer);
         case "type": return try parseObjectTypeDefinition(lexer: lexer);
diff --git a/Sources/GraphQL/Map/AnySerialization.swift b/Sources/GraphQL/Map/AnySerialization.swift
index 695c074c..da30a51c 100644
--- a/Sources/GraphQL/Map/AnySerialization.swift
+++ b/Sources/GraphQL/Map/AnySerialization.swift
@@ -6,6 +6,15 @@ public struct AnySerialization {
     }
     
     static func object(with map: Any) throws -> NSObject {
-        return map as! NSObject
+        guard let result = map as? NSObject else {
+            throw EncodingError.invalidValue(
+                map,
+                EncodingError.Context(
+                    codingPath: [],
+                    debugDescription: "Expected object input to be castable to NSObject: \(type(of: map))"
+                )
+            )
+        }
+        return result
     }
 }
diff --git a/Sources/GraphQL/Map/Map.swift b/Sources/GraphQL/Map/Map.swift
index 9a471f71..4e8b3d40 100644
--- a/Sources/GraphQL/Map/Map.swift
+++ b/Sources/GraphQL/Map/Map.swift
@@ -663,7 +663,13 @@ extension Map : Codable {
         
         switch self {
         case .undefined:
-            fatalError("undefined values should have been excluded from encoding")
+            throw EncodingError.invalidValue(
+                self,
+                EncodingError.Context(
+                    codingPath: [],
+                    debugDescription: "undefined values should have been excluded from encoding"
+                )
+            )
         case .null:
             try container.encodeNil()
         case let .bool(value):
@@ -681,7 +687,15 @@ extension Map : Codable {
             var container = encoder.container(keyedBy: _DictionaryCodingKey.self)
             for (key, value) in dictionary {
                 if !value.isUndefined {
-                    let codingKey = _DictionaryCodingKey(stringValue: key)!
+                    guard let codingKey = _DictionaryCodingKey(stringValue: key) else {
+                        throw EncodingError.invalidValue(
+                            self,
+                            EncodingError.Context(
+                                codingPath: [],
+                                debugDescription: "codingKey not found for dictionary key: \(key)"
+                            )
+                        )
+                    }
                     try container.encode(value, forKey: codingKey)
                 }
             }
diff --git a/Sources/GraphQL/Map/MapSerialization.swift b/Sources/GraphQL/Map/MapSerialization.swift
index 5a2ea022..76779dee 100644
--- a/Sources/GraphQL/Map/MapSerialization.swift
+++ b/Sources/GraphQL/Map/MapSerialization.swift
@@ -12,13 +12,40 @@ public struct MapSerialization {
             return .string(string as String)
         case let array as NSArray:
             let array: [Map] = try array.map { value in
-                try self.map(with: value as! NSObject)
+                guard let value = value as? NSObject else {
+                    throw EncodingError.invalidValue(
+                        array,
+                        EncodingError.Context(
+                            codingPath: [],
+                            debugDescription: "Array value was not an object: \(value) in \(array)"
+                        )
+                    )
+                }
+                return try self.map(with: value)
             }
             return .array(array)
         case let dictionary as NSDictionary:
             // Extract from an unordered dictionary, using NSDictionary extraction order
             let orderedDictionary: OrderedDictionary<String, Map> = try dictionary.reduce(into: [:]) { (dictionary, pair) in
-                dictionary[pair.key as! String] = try self.map(with: pair.value as! NSObject)
+                guard let key = pair.key as? String else {
+                    throw EncodingError.invalidValue(
+                        dictionary,
+                        EncodingError.Context(
+                            codingPath: [],
+                            debugDescription: "Dictionary key was not string: \(pair.key) in \(dictionary)"
+                        )
+                    )
+                }
+                guard let value = pair.value as? NSObject else{
+                    throw EncodingError.invalidValue(
+                        dictionary,
+                        EncodingError.Context(
+                            codingPath: [],
+                            debugDescription: "Dictionary value was not an object: \(key) in \(dictionary)"
+                        )
+                    )
+                }
+                dictionary[key] = try self.map(with: value)
             }
             return .dictionary(orderedDictionary)
         default:
@@ -35,7 +62,13 @@ public struct MapSerialization {
     static func object(with map: Map) throws -> NSObject {
         switch map {
         case .undefined:
-            fatalError("undefined values should have been excluded from serialization")
+            throw EncodingError.invalidValue(
+                self,
+                EncodingError.Context(
+                    codingPath: [],
+                    debugDescription: "undefined values should have been excluded from serialization"
+                )
+            )
         case .null:
             return NSNull()
         case let .bool(value):
diff --git a/Sources/GraphQL/Subscription/Subscribe.swift b/Sources/GraphQL/Subscription/Subscribe.swift
index d10076c0..b1428a4b 100644
--- a/Sources/GraphQL/Subscription/Subscribe.swift
+++ b/Sources/GraphQL/Subscription/Subscribe.swift
@@ -178,10 +178,15 @@ func executeSubscription(
         fields: &inputFields,
         visitedFragmentNames: &visitedFragmentNames
     )
-    // If query is valid, fields is guaranteed to have at least 1 member
-    let responseName = fields.keys.first!
-    let fieldNodes = fields[responseName]!
-    let fieldNode = fieldNodes.first!
+    
+    // If query is valid, fields should have at least 1 member
+    guard let responseName = fields.keys.first,
+          let fieldNodes = fields[responseName],
+          let fieldNode = fieldNodes.first else {
+        throw GraphQLError(
+            message: "Subscription field resolution resulted in no field nodes."
+        )
+    }
     
     guard let fieldDef = getFieldDef(schema: context.schema, parentType: type, fieldAST: fieldNode) else {
         throw GraphQLError(
diff --git a/Sources/GraphQL/SwiftUtilities/SuggestionList.swift b/Sources/GraphQL/SwiftUtilities/SuggestionList.swift
index 9b190adf..cc3aa228 100644
--- a/Sources/GraphQL/SwiftUtilities/SuggestionList.swift
+++ b/Sources/GraphQL/SwiftUtilities/SuggestionList.swift
@@ -20,6 +20,7 @@ func suggestionList(
 
     }
     return optionsByDistance.keys.sorted {
+        // Values are guaranteed non-nil since the keys come from the object itself
         optionsByDistance[$0]! - optionsByDistance[$1]! != 0
     }
 }
diff --git a/Sources/GraphQL/Type/Schema.swift b/Sources/GraphQL/Type/Schema.swift
index 0fff7452..71a6ea94 100644
--- a/Sources/GraphQL/Type/Schema.swift
+++ b/Sources/GraphQL/Type/Schema.swift
@@ -108,7 +108,11 @@ public final class GraphQLSchema {
     public func getImplementations(
         interfaceType: GraphQLInterfaceType
     ) -> InterfaceImplementations {
-        implementations[interfaceType.name]!
+        guard let matchingImplementations = implementations[interfaceType.name] else {
+            // If we ask for an interface that hasn't been defined, just return no types.
+            return InterfaceImplementations.init()
+        }
+        return matchingImplementations
     }
 
     // @deprecated: use isSubType instead - will be removed in the future.
diff --git a/Sources/GraphQL/Utilities/ASTFromValue.swift b/Sources/GraphQL/Utilities/ASTFromValue.swift
index 71095b6e..4009d986 100644
--- a/Sources/GraphQL/Utilities/ASTFromValue.swift
+++ b/Sources/GraphQL/Utilities/ASTFromValue.swift
@@ -19,9 +19,12 @@ func astFromValue(
     type: GraphQLInputType
 ) throws -> Value? {
     if let type = type as? GraphQLNonNull {
-        // Note: we're not checking that the result is non-null.
-        // This function is not responsible for validating the input value.
-        return try astFromValue(value: value, type: type.ofType as! GraphQLInputType)
+        guard let nonNullType = type.ofType as? GraphQLInputType else {
+            throw GraphQLError(
+                message: "Expected GraphQLNonNull to contain an input type \(type)"
+            )
+        }
+        return try astFromValue(value: value, type: nonNullType)
     }
 
     guard value != .null else {
@@ -31,7 +34,11 @@ func astFromValue(
     // Convert array to GraphQL list. If the GraphQLType is a list, but
     // the value is not an array, convert the value using the list's item type.
     if let type = type as? GraphQLList {
-        let itemType = type.ofType as! GraphQLInputType
+        guard let itemType = type.ofType as? GraphQLInputType else {
+            throw GraphQLError(
+                message: "Expected GraphQLList to contain an input type \(type)"
+            )
+        }
 
         if case .array(let value) = value {
             var valuesASTs: [Value] = []
@@ -72,7 +79,7 @@ func astFromValue(
 
     guard let leafType = type as? GraphQLLeafType else {
         throw GraphQLError(
-            message: "Must provide Input Type, cannot use: \(type)"
+            message: "Expected scalar non-object type to be a leaf type: \(type)"
         )
     }
 
@@ -116,7 +123,11 @@ func astFromValue(
         }
         
         let data = try GraphQLJSONEncoder().encode(Wrapper(map: serialized))
-        let string = String(data: data, encoding: .utf8)!
+        guard let string = String(data: data, encoding: .utf8) else {
+            throw GraphQLError(
+                message: "Unable to convert data to utf8 string: \(data)"
+            )
+        }
         return StringValue(value: String(string.dropFirst(8).dropLast(2)))
     }
     
diff --git a/Sources/GraphQL/Utilities/NIO+Extensions.swift b/Sources/GraphQL/Utilities/NIO+Extensions.swift
index 4f5b200d..89da46b1 100644
--- a/Sources/GraphQL/Utilities/NIO+Extensions.swift
+++ b/Sources/GraphQL/Utilities/NIO+Extensions.swift
@@ -51,6 +51,7 @@ extension OrderedDictionary where Value : FutureType {
         return EventLoopFuture.whenAllSucceed(futures, on: eventLoopGroup.next()).map { unorderedResult in
             var result: OrderedDictionary<Key, Value.Expectation> = [:]
             for key in keys {
+                // Unwrap is guaranteed because keys are from original dictionary and maps preserve all elements
                 result[key] = unorderedResult.first(where: { $0.0 == key })!.1
             }
             return result
diff --git a/Sources/GraphQL/Utilities/TypeFromAST.swift b/Sources/GraphQL/Utilities/TypeFromAST.swift
index 39501ad8..a90cf735 100644
--- a/Sources/GraphQL/Utilities/TypeFromAST.swift
+++ b/Sources/GraphQL/Utilities/TypeFromAST.swift
@@ -7,6 +7,7 @@ func typeFromAST(schema: GraphQLSchema, inputTypeAST: Type) -> GraphQLType? {
     
     if let nonNullType = inputTypeAST as? NonNullType {
         if let innerType = typeFromAST(schema: schema, inputTypeAST: nonNullType.type) {
+            // Non-null types by definition must contain nullable types (since all types are nullable by default)
             return GraphQLNonNull(innerType as! GraphQLNullableType)
         }
     }
diff --git a/Sources/GraphQL/Utilities/TypeInfo.swift b/Sources/GraphQL/Utilities/TypeInfo.swift
index 1483e69c..27ea67bf 100644
--- a/Sources/GraphQL/Utilities/TypeInfo.swift
+++ b/Sources/GraphQL/Utilities/TypeInfo.swift
@@ -90,8 +90,12 @@ final class TypeInfo {
             typeStack.append(type)
 
         case let node as InlineFragment:
-            let typeConditionAST = node.typeCondition
-            let outputType = typeConditionAST != nil ? typeFromAST(schema: schema, inputTypeAST: typeConditionAST!) : self.type
+            let outputType: GraphQLType?
+            if let typeConditionAST = node.typeCondition {
+                outputType = typeFromAST(schema: schema, inputTypeAST: typeConditionAST)
+            } else {
+                outputType = self.type
+            }
             typeStack.append(outputType as? GraphQLOutputType)
 
         case let node as FragmentDefinition:
diff --git a/Sources/GraphQL/Validation/Rules/FieldsOnCorrectTypeRule.swift b/Sources/GraphQL/Validation/Rules/FieldsOnCorrectTypeRule.swift
index 0cb46a24..71c2af90 100644
--- a/Sources/GraphQL/Validation/Rules/FieldsOnCorrectTypeRule.swift
+++ b/Sources/GraphQL/Validation/Rules/FieldsOnCorrectTypeRule.swift
@@ -102,6 +102,7 @@ func getSuggestedTypeNames(
 
         // Suggest interface types based on how common they are.
         let suggestedInterfaceTypes = interfaceUsageCount.keys.sorted {
+            // Unwraps are safe because keys are from object being sorted.
             interfaceUsageCount[$1]! - interfaceUsageCount[$0]! >= 0
         }
 
diff --git a/Sources/GraphQL/Validation/Validate.swift b/Sources/GraphQL/Validation/Validate.swift
index edb0055b..38629264 100644
--- a/Sources/GraphQL/Validation/Validate.swift
+++ b/Sources/GraphQL/Validation/Validate.swift
@@ -153,105 +153,100 @@ public final class ValidationContext {
     }
 
     public func getFragmentSpreads(node: SelectionSet) -> [FragmentSpread] {
-        var spreads = fragmentSpreads[node]
+        if let spreads = fragmentSpreads[node] {
+            return spreads
+        }
 
-        if spreads == nil {
-            spreads = []
-            var setsToVisit: [SelectionSet] = [node]
+        var spreads = [FragmentSpread]()
+        var setsToVisit: [SelectionSet] = [node]
 
-            while let set = setsToVisit.popLast() {
-                for selection in set.selections {
-                    if let selection = selection as? FragmentSpread {
-                        spreads!.append(selection)
-                    }
+        while let set = setsToVisit.popLast() {
+            for selection in set.selections {
+                if let selection = selection as? FragmentSpread {
+                    spreads.append(selection)
+                }
 
-                    if let selection = selection as? InlineFragment {
-                        setsToVisit.append(selection.selectionSet)
-                    }
+                if let selection = selection as? InlineFragment {
+                    setsToVisit.append(selection.selectionSet)
+                }
 
-                    if let selection = selection as? Field, let selectionSet = selection.selectionSet {
-                        setsToVisit.append(selectionSet)
-                    }
+                if let selection = selection as? Field, let selectionSet = selection.selectionSet {
+                    setsToVisit.append(selectionSet)
                 }
             }
-
-            fragmentSpreads[node] = spreads
         }
 
-        return spreads!
+        fragmentSpreads[node] = spreads
+        return spreads
     }
 
     public func getRecursivelyReferencedFragments(operation: OperationDefinition) -> [FragmentDefinition] {
-        var fragments = recursivelyReferencedFragments[operation]
-
-        if fragments == nil {
-            fragments = []
-            var collectedNames: [String: Bool] = [:]
-            var nodesToVisit: [SelectionSet] = [operation.selectionSet]
-
-            while let node = nodesToVisit.popLast() {
-                let spreads = getFragmentSpreads(node: node)
-
-                for spread in spreads {
-                    let fragName = spread.name.value
-                    if collectedNames[fragName] != true {
-                        collectedNames[fragName] = true
-                        if let fragment = getFragment(name: fragName) {
-                            fragments!.append(fragment)
-                            nodesToVisit.append(fragment.selectionSet)
-                        }
+        if let fragments = recursivelyReferencedFragments[operation] {
+            return fragments
+        }
+        
+        var fragments = [FragmentDefinition]()
+        var collectedNames: [String: Bool] = [:]
+        var nodesToVisit: [SelectionSet] = [operation.selectionSet]
+
+        while let node = nodesToVisit.popLast() {
+            let spreads = getFragmentSpreads(node: node)
+
+            for spread in spreads {
+                let fragName = spread.name.value
+                if collectedNames[fragName] != true {
+                    collectedNames[fragName] = true
+                    if let fragment = getFragment(name: fragName) {
+                        fragments.append(fragment)
+                        nodesToVisit.append(fragment.selectionSet)
                     }
                 }
             }
-            
-            recursivelyReferencedFragments[operation] = fragments
         }
         
-        return fragments!
+        recursivelyReferencedFragments[operation] = fragments
+        return fragments
     }
 
     public func getVariableUsages(node: HasSelectionSet) -> [VariableUsage] {
-        var usages = variableUsages[node]
-
-        if usages == nil {
-            var newUsages: [VariableUsage] = []
-            let typeInfo = TypeInfo(schema: schema)
-
-            visit(root: node.node, visitor: visitWithTypeInfo(typeInfo: typeInfo, visitor: Visitor(enter: { node, _, _, _, _ in
-                if node is VariableDefinition {
-                    return .skip
-                }
+        if let usages = variableUsages[node] {
+            return usages
+        }
+        
+        var usages = [VariableUsage]()
+        let typeInfo = TypeInfo(schema: schema)
 
-                if let variable = node as? Variable {
-                    newUsages.append(VariableUsage(node: variable, type: typeInfo.inputType))
-                }
+        visit(root: node.node, visitor: visitWithTypeInfo(typeInfo: typeInfo, visitor: Visitor(enter: { node, _, _, _, _ in
+            if node is VariableDefinition {
+                return .skip
+            }
 
-                return .continue
-            })))
+            if let variable = node as? Variable {
+                usages.append(VariableUsage(node: variable, type: typeInfo.inputType))
+            }
 
-            usages = newUsages
-            variableUsages[node] = usages
-        }
+            return .continue
+        })))
 
-        return usages!
+        variableUsages[node] = usages
+        return usages
     }
 
     public func getRecursiveVariableUsages(operation: OperationDefinition) -> [VariableUsage] {
-        var usages = recursiveVariableUsages[operation]
-
-        if usages == nil {
-            usages = getVariableUsages(node: .operation(operation))
-            let fragments = getRecursivelyReferencedFragments(operation: operation)
-
-            for fragment in fragments {
-                let newUsages = getVariableUsages(node: .fragment(fragment))
-                usages!.append(contentsOf: newUsages)
-            }
-
-            recursiveVariableUsages[operation] = usages
+        if let usages = recursiveVariableUsages[operation] {
+            return usages
         }
         
-        return usages!
+        var usages = getVariableUsages(node: .operation(operation))
+        let fragments = getRecursivelyReferencedFragments(operation: operation)
+
+        for fragment in fragments {
+            let newUsages = getVariableUsages(node: .fragment(fragment))
+            usages.append(contentsOf: newUsages)
+        }
+
+        recursiveVariableUsages[operation] = usages
+        return usages
     }
 
     public var type: GraphQLOutputType? {
diff --git a/Tests/GraphQLTests/InputTests/InputTests.swift b/Tests/GraphQLTests/InputTests/InputTests.swift
index c9969973..20b0fbaa 100644
--- a/Tests/GraphQLTests/InputTests/InputTests.swift
+++ b/Tests/GraphQLTests/InputTests/InputTests.swift
@@ -14,7 +14,7 @@ class InputTests : XCTestCase {
             let field1: String
         }
         
-        let EchoOutputType = try! GraphQLObjectType(
+        let EchoOutputType = try GraphQLObjectType(
             name: "Echo",
             description: "",
             fields: [
@@ -27,8 +27,8 @@ class InputTests : XCTestCase {
             }
         )
         
-        let schema = try! GraphQLSchema(
-            query: try! GraphQLObjectType(
+        let schema = try GraphQLSchema(
+            query: try GraphQLObjectType(
                 name: "Query",
                 fields: [
                     "echo": GraphQLField(
@@ -180,7 +180,7 @@ class InputTests : XCTestCase {
             let field1: String?
         }
         
-        let EchoOutputType = try! GraphQLObjectType(
+        let EchoOutputType = try GraphQLObjectType(
             name: "Echo",
             description: "",
             fields: [
@@ -193,8 +193,8 @@ class InputTests : XCTestCase {
             }
         )
         
-        let schema = try! GraphQLSchema(
-            query: try! GraphQLObjectType(
+        let schema = try GraphQLSchema(
+            query: try GraphQLObjectType(
                 name: "Query",
                 fields: [
                     "echo": GraphQLField(
@@ -362,7 +362,7 @@ class InputTests : XCTestCase {
             let field1: String
         }
         
-        let EchoOutputType = try! GraphQLObjectType(
+        let EchoOutputType = try GraphQLObjectType(
             name: "Echo",
             description: "",
             fields: [
@@ -375,8 +375,8 @@ class InputTests : XCTestCase {
             }
         )
         
-        let schema = try! GraphQLSchema(
-            query: try! GraphQLObjectType(
+        let schema = try GraphQLSchema(
+            query: try GraphQLObjectType(
                 name: "Query",
                 fields: [
                     "echo": GraphQLField(
@@ -556,7 +556,7 @@ class InputTests : XCTestCase {
             let field1: String?
         }
         
-        let EchoOutputType = try! GraphQLObjectType(
+        let EchoOutputType = try GraphQLObjectType(
             name: "Echo",
             description: "",
             fields: [
@@ -569,8 +569,8 @@ class InputTests : XCTestCase {
             }
         )
         
-        let schema = try! GraphQLSchema(
-            query: try! GraphQLObjectType(
+        let schema = try GraphQLSchema(
+            query: try GraphQLObjectType(
                 name: "Query",
                 fields: [
                     "echo": GraphQLField(
@@ -777,7 +777,7 @@ class InputTests : XCTestCase {
             let input: Echo
         }
 
-        let EchoInputType = try! GraphQLInputObjectType(
+        let EchoInputType = try GraphQLInputObjectType(
             name: "EchoInput",
             fields: [
                 "field1": InputObjectField(
@@ -789,7 +789,7 @@ class InputTests : XCTestCase {
             ]
         )
 
-        let EchoOutputType = try! GraphQLObjectType(
+        let EchoOutputType = try GraphQLObjectType(
             name: "Echo",
             description: "",
             fields: [
@@ -805,8 +805,8 @@ class InputTests : XCTestCase {
             }
         )
         
-        let schema = try! GraphQLSchema(
-            query: try! GraphQLObjectType(
+        let schema = try GraphQLSchema(
+            query: try GraphQLObjectType(
                 name: "Query",
                 fields: [
                     "echo": GraphQLField(
@@ -912,7 +912,7 @@ class InputTests : XCTestCase {
             let input: Echo
         }
 
-        let EchoInputType = try! GraphQLInputObjectType(
+        let EchoInputType = try GraphQLInputObjectType(
             name: "EchoInput",
             fields: [
                 "field1": InputObjectField(
@@ -924,7 +924,7 @@ class InputTests : XCTestCase {
             ]
         )
 
-        let EchoOutputType = try! GraphQLObjectType(
+        let EchoOutputType = try GraphQLObjectType(
             name: "Echo",
             description: "",
             fields: [
@@ -940,8 +940,8 @@ class InputTests : XCTestCase {
             }
         )
         
-        let schema = try! GraphQLSchema(
-            query: try! GraphQLObjectType(
+        let schema = try GraphQLSchema(
+            query: try GraphQLObjectType(
                 name: "Query",
                 fields: [
                     "echo": GraphQLField(
@@ -1047,7 +1047,7 @@ class InputTests : XCTestCase {
             let input: Echo
         }
 
-        let EchoInputType = try! GraphQLInputObjectType(
+        let EchoInputType = try GraphQLInputObjectType(
             name: "EchoInput",
             fields: [
                 "field1": InputObjectField(
@@ -1059,7 +1059,7 @@ class InputTests : XCTestCase {
             ]
         )
 
-        let EchoOutputType = try! GraphQLObjectType(
+        let EchoOutputType = try GraphQLObjectType(
             name: "Echo",
             description: "",
             fields: [
@@ -1075,8 +1075,8 @@ class InputTests : XCTestCase {
             }
         )
         
-        let schema = try! GraphQLSchema(
-            query: try! GraphQLObjectType(
+        let schema = try GraphQLSchema(
+            query: try GraphQLObjectType(
                 name: "Query",
                 fields: [
                     "echo": GraphQLField(
@@ -1180,7 +1180,7 @@ class InputTests : XCTestCase {
             let input: Echo
         }
 
-        let EchoInputType = try! GraphQLInputObjectType(
+        let EchoInputType = try GraphQLInputObjectType(
             name: "EchoInput",
             fields: [
                 "field1": InputObjectField(
@@ -1193,7 +1193,7 @@ class InputTests : XCTestCase {
             ]
         )
 
-        let EchoOutputType = try! GraphQLObjectType(
+        let EchoOutputType = try GraphQLObjectType(
             name: "Echo",
             description: "",
             fields: [
@@ -1209,8 +1209,8 @@ class InputTests : XCTestCase {
             }
         )
         
-        let schema = try! GraphQLSchema(
-            query: try! GraphQLObjectType(
+        let schema = try GraphQLSchema(
+            query: try GraphQLObjectType(
                 name: "Query",
                 fields: [
                     "echo": GraphQLField(
diff --git a/Tests/GraphQLTests/LanguageTests/SchemaParserTests.swift b/Tests/GraphQLTests/LanguageTests/SchemaParserTests.swift
index fa249e4b..3a31e617 100644
--- a/Tests/GraphQLTests/LanguageTests/SchemaParserTests.swift
+++ b/Tests/GraphQLTests/LanguageTests/SchemaParserTests.swift
@@ -428,7 +428,8 @@ class SchemaParserTests : XCTestCase {
         )
         
         let result = try parse(source: source)
-        XCTAssertEqual(result.definitions[0] as! ObjectTypeDefinition, expected, "\n\(dump(result.definitions[0]))\n\(dump(expected))\n")
+        let firstDefinition = try XCTUnwrap(result.definitions[0] as? ObjectTypeDefinition)
+        XCTAssertEqual(firstDefinition, expected, "\n\(dump(firstDefinition))\n\(dump(expected))\n")
     }
     
     func testTypeWitMultilinehDescription() throws {
diff --git a/Tests/GraphQLTests/SubscriptionTests/SubscriptionSchema.swift b/Tests/GraphQLTests/SubscriptionTests/SubscriptionSchema.swift
index bbe1894d..0b2eef67 100644
--- a/Tests/GraphQLTests/SubscriptionTests/SubscriptionSchema.swift
+++ b/Tests/GraphQLTests/SubscriptionTests/SubscriptionSchema.swift
@@ -119,8 +119,8 @@ class EmailDb {
     }
     
     /// Returns the default email schema, with standard resolvers.
-    func defaultSchema() -> GraphQLSchema {
-        return emailSchemaWithResolvers(
+    func defaultSchema() throws -> GraphQLSchema {
+        return try emailSchemaWithResolvers(
             resolve: {emailAny, _, _, eventLoopGroup, _ throws -> EventLoopFuture<Any?> in
                 if let email = emailAny as? Email {
                     return eventLoopGroup.next().makeSucceededFuture(EmailEvent(
@@ -155,8 +155,8 @@ class EmailDb {
 }
 
 /// Generates an email schema with the specified resolve and subscribe methods
-func emailSchemaWithResolvers(resolve: GraphQLFieldResolve? = nil, subscribe: GraphQLFieldResolve? = nil) -> GraphQLSchema {
-    return try! GraphQLSchema(
+func emailSchemaWithResolvers(resolve: GraphQLFieldResolve? = nil, subscribe: GraphQLFieldResolve? = nil) throws -> GraphQLSchema {
+    return try GraphQLSchema(
         query: EmailQueryType,
         subscription: try! GraphQLObjectType(
             name: "Subscription",
diff --git a/Tests/GraphQLTests/SubscriptionTests/SubscriptionTests.swift b/Tests/GraphQLTests/SubscriptionTests/SubscriptionTests.swift
index fbb790c4..44db10b9 100644
--- a/Tests/GraphQLTests/SubscriptionTests/SubscriptionTests.swift
+++ b/Tests/GraphQLTests/SubscriptionTests/SubscriptionTests.swift
@@ -15,7 +15,7 @@ class SubscriptionTests : XCTestCase {
     /// This test is not present in graphql-js, but just tests basic functionality.
     func testGraphqlSubscribe() async throws {
         let db = EmailDb()
-        let schema = db.defaultSchema()
+        let schema = try db.defaultSchema()
         let query = """
             subscription ($priority: Int = 0) {
                 importantEmail(priority: $priority) {
@@ -78,7 +78,7 @@ class SubscriptionTests : XCTestCase {
         let db = EmailDb()
         let schema = try GraphQLSchema(
             query: EmailQueryType,
-            subscription: try! GraphQLObjectType(
+            subscription: GraphQLObjectType(
                 name: "Subscription",
                 fields: [
                     "importantEmail": GraphQLField(
@@ -180,7 +180,7 @@ class SubscriptionTests : XCTestCase {
 
         let schema = try GraphQLSchema(
             query: EmailQueryType,
-            subscription: try! GraphQLObjectType(
+            subscription: GraphQLObjectType(
                 name: "Subscription",
                 fields: [
                     "importantEmail": GraphQLField(
@@ -270,7 +270,7 @@ class SubscriptionTests : XCTestCase {
 
     /// 'throws an error if subscribe does not return an iterator'
     func testErrorIfSubscribeIsntIterator() throws {
-        let schema = emailSchemaWithResolvers(
+        let schema = try emailSchemaWithResolvers(
             resolve: {_, _, _, eventLoopGroup, _ throws -> EventLoopFuture<Any?> in
                 return eventLoopGroup.next().makeSucceededFuture(nil)
             },
@@ -323,21 +323,21 @@ class SubscriptionTests : XCTestCase {
         }
 
         // Throwing an error
-        verifyError(schema: emailSchemaWithResolvers(
+        try verifyError(schema: emailSchemaWithResolvers(
             subscribe: {_, _, _, eventLoopGroup, _ throws -> EventLoopFuture<Any?> in
                 throw GraphQLError(message: "test error")
             }
         ))
 
         // Resolving to an error
-        verifyError(schema: emailSchemaWithResolvers(
+        try verifyError(schema: emailSchemaWithResolvers(
             subscribe: {_, _, _, eventLoopGroup, _ throws -> EventLoopFuture<Any?> in
                 return eventLoopGroup.next().makeSucceededFuture(GraphQLError(message: "test error"))
             }
         ))
 
         // Rejecting with an error
-        verifyError(schema: emailSchemaWithResolvers(
+        try verifyError(schema: emailSchemaWithResolvers(
             subscribe: {_, _, _, eventLoopGroup, _ throws -> EventLoopFuture<Any?> in
                 return eventLoopGroup.next().makeFailedFuture(GraphQLError(message: "test error"))
             }
@@ -837,7 +837,7 @@ class SubscriptionTests : XCTestCase {
     func testErrorDuringSubscription() async throws {
         let db = EmailDb()
 
-        let schema = emailSchemaWithResolvers(
+        let schema = try emailSchemaWithResolvers(
             resolve: {emailAny, _, _, eventLoopGroup, _ throws -> EventLoopFuture<Any?> in
                 guard let email = emailAny as? Email else {
                     throw GraphQLError(message:"Source is not Email type: \(type(of: emailAny))")