diff --git a/gql/utilities/serialize_variable_values.py b/gql/utilities/serialize_variable_values.py
index 833df8bd..38ad1995 100644
--- a/gql/utilities/serialize_variable_values.py
+++ b/gql/utilities/serialize_variable_values.py
@@ -85,6 +85,7 @@ def serialize_value(type_: GraphQLType, value: Any) -> Any:
         return {
             field_name: serialize_value(field.type, value[field_name])
             for field_name, field in type_.fields.items()
+            if field_name in value
         }
 
     raise GraphQLError(f"Impossible to serialize value with type: {inspect(type_)}.")
diff --git a/tests/custom_scalars/test_datetime.py b/tests/custom_scalars/test_datetime.py
index 169ce076..b3e717c5 100644
--- a/tests/custom_scalars/test_datetime.py
+++ b/tests/custom_scalars/test_datetime.py
@@ -75,7 +75,10 @@ def resolve_seconds(root, _info, interval):
 IntervalInputType = GraphQLInputObjectType(
     "IntervalInput",
     fields={
-        "start": GraphQLInputField(DatetimeScalar),
+        "start": GraphQLInputField(
+            DatetimeScalar,
+            default_value=datetime(2021, 11, 12, 11, 58, 13, 461161),
+        ),
         "end": GraphQLInputField(DatetimeScalar),
     },
 )
@@ -216,3 +219,26 @@ def test_seconds():
     print(result)
 
     assert result["seconds"] == 432000
+
+
+@pytest.mark.skipif(
+    not hasattr(datetime, "fromisoformat"), reason="fromisoformat is new in Python 3.7+"
+)
+def test_seconds_omit_optional_start_argument():
+    client = Client(schema=schema)
+
+    in_five_days = datetime.fromisoformat("2021-11-17T11:58:13.461161")
+
+    query = gql(
+        "query seconds($interval: IntervalInput) {seconds(interval: $interval)}"
+    )
+
+    variable_values = {"interval": {"end": in_five_days}}
+
+    result = client.execute(
+        query, variable_values=variable_values, serialize_variables=True
+    )
+
+    print(result)
+
+    assert result["seconds"] == 432000