diff --git a/src/HttpMethodsClient.php b/src/HttpMethodsClient.php
index 497921b..95fee3b 100644
--- a/src/HttpMethodsClient.php
+++ b/src/HttpMethodsClient.php
@@ -33,7 +33,7 @@ final class HttpMethodsClient implements HttpMethodsClientInterface
     /**
      * @param RequestFactory|RequestFactoryInterface $requestFactory
      */
-    public function __construct(ClientInterface $httpClient, $requestFactory, StreamFactoryInterface $streamFactory = null)
+    public function __construct(ClientInterface $httpClient, $requestFactory, ?StreamFactoryInterface $streamFactory = null)
     {
         if (!$requestFactory instanceof RequestFactory && !$requestFactory instanceof RequestFactoryInterface) {
             throw new \TypeError(