diff --git a/src/CachedKeySet.php b/src/CachedKeySet.php
index baf801f1..ee529f9f 100644
--- a/src/CachedKeySet.php
+++ b/src/CachedKeySet.php
@@ -178,6 +178,16 @@ private function keyIdExists(string $keyId): bool
             }
             $request = $this->httpFactory->createRequest('GET', $this->jwksUri);
             $jwksResponse = $this->httpClient->sendRequest($request);
+            if ($jwksResponse->getStatusCode() !== 200) {
+                throw new UnexpectedValueException(
+                    sprintf('HTTP Error: %d %s for URI "%s"',
+                        $jwksResponse->getStatusCode(),
+                        $jwksResponse->getReasonPhrase(),
+                        $this->jwksUri,
+                    ),
+                    $jwksResponse->getStatusCode()
+                );
+            }
             $this->keySet = $this->formatJwksForCache((string) $jwksResponse->getBody());
 
             if (!isset($this->keySet[$keyId])) {
diff --git a/tests/CachedKeySetTest.php b/tests/CachedKeySetTest.php
index 9142fda6..1e73af6d 100644
--- a/tests/CachedKeySetTest.php
+++ b/tests/CachedKeySetTest.php
@@ -88,6 +88,37 @@ public function testOutOfBoundsThrowsException()
         $cachedKeySet['bar'];
     }
 
+    public function testInvalidHttpResponseThrowsException()
+    {
+        $this->expectException(\UnexpectedValueException::class);
+        $this->expectExceptionMessage('HTTP Error: 404 URL not found');
+        $this->expectExceptionCode(404);
+
+        $body = $this->prophesize('Psr\Http\Message\StreamInterface');
+
+        $response = $this->prophesize('Psr\Http\Message\ResponseInterface');
+        $response->getStatusCode()
+            ->shouldBeCalled()
+            ->willReturn(404);
+        $response->getReasonPhrase()
+            ->shouldBeCalledTimes(1)
+            ->willReturn('URL not found');
+
+        $http = $this->prophesize(ClientInterface::class);
+        $http->sendRequest(Argument::any())
+            ->shouldBeCalledTimes(1)
+            ->willReturn($response->reveal());
+
+        $cachedKeySet = new CachedKeySet(
+            $this->testJwksUri,
+            $http->reveal(),
+            $this->getMockHttpFactory(),
+            $this->getMockEmptyCache()
+        );
+
+        isset($cachedKeySet[0]);
+    }
+
     public function testWithExistingKeyId()
     {
         $cachedKeySet = new CachedKeySet(
@@ -382,6 +413,9 @@ private function getMockHttpClient($testJwks, int $timesCalled = 1)
         $response->getBody()
             ->shouldBeCalledTimes($timesCalled)
             ->willReturn($body->reveal());
+        $response->getStatusCode()
+            ->shouldBeCalledTimes($timesCalled)
+            ->willReturn(200);
 
         $http = $this->prophesize(ClientInterface::class);
         $http->sendRequest(Argument::any())