diff --git a/src/Client.php b/src/Client.php
index 4287ccc..fac3532 100644
--- a/src/Client.php
+++ b/src/Client.php
@@ -26,7 +26,7 @@ class Client implements HttpClient, HttpAsyncClient
      * Initialize the React client.
      */
     public function __construct(
-        ReactBrowser $client = null
+        ?ReactBrowser $client = null
     ) {
         $this->client = $client ?: ReactFactory::buildHttpClient();
     }
diff --git a/src/ReactFactory.php b/src/ReactFactory.php
index 10059a6..3e274e8 100644
--- a/src/ReactFactory.php
+++ b/src/ReactFactory.php
@@ -19,7 +19,7 @@ class ReactFactory
      *                                           behaviour
      */
     public static function buildHttpClient(
-        ConnectorInterface $connector = null
+        ?ConnectorInterface $connector = null
     ): Browser {
         return (new Browser($connector))
             ->withRejectErrorResponse(false)