diff --git a/lib/promise-adapter/src/Adapter/WebonyxGraphQLSyncPromiseAdapter.php b/lib/promise-adapter/src/Adapter/WebonyxGraphQLSyncPromiseAdapter.php
index 849ec60..dbf8a65 100644
--- a/lib/promise-adapter/src/Adapter/WebonyxGraphQLSyncPromiseAdapter.php
+++ b/lib/promise-adapter/src/Adapter/WebonyxGraphQLSyncPromiseAdapter.php
@@ -30,7 +30,7 @@ class WebonyxGraphQLSyncPromiseAdapter implements PromiseAdapterInterface
      */
     private $webonyxPromiseAdapter;
 
-    public function __construct(SyncPromiseAdapter $webonyxPromiseAdapter = null)
+    public function __construct(?SyncPromiseAdapter $webonyxPromiseAdapter = null)
     {
         $webonyxPromiseAdapter = $webonyxPromiseAdapter?:new SyncPromiseAdapter();
         $this->setWebonyxPromiseAdapter($webonyxPromiseAdapter);