diff --git a/src/Runtime/Client/EndpointTrait.php b/src/Runtime/Client/EndpointTrait.php
index 3747e267..38fbca15 100644
--- a/src/Runtime/Client/EndpointTrait.php
+++ b/src/Runtime/Client/EndpointTrait.php
@@ -23,5 +23,5 @@ public function parseResponse(ResponseInterface $response, SerializerInterface $
         throw new InvalidFetchModeException(sprintf('Fetch mode %s is not supported', $fetchMode));
     }
 
-    abstract protected function transformResponseBody(string $body, int $status, SerializerInterface $serializer, string $contentType = null);
+    abstract protected function transformResponseBody(string $body, int $status, SerializerInterface $serializer, string $contentType);
 }