diff --git a/src/Promise/Adapter/Webonyx/GraphQL/SyncPromiseAdapter.php b/src/Promise/Adapter/Webonyx/GraphQL/SyncPromiseAdapter.php
index fc2f82a..da42184 100644
--- a/src/Promise/Adapter/Webonyx/GraphQL/SyncPromiseAdapter.php
+++ b/src/Promise/Adapter/Webonyx/GraphQL/SyncPromiseAdapter.php
@@ -17,12 +17,12 @@
 
 class SyncPromiseAdapter extends BaseSyncPromiseAdapter
 {
-    protected function beforeWait(Promise $promise)
+    protected function beforeWait(Promise $promise): void
     {
         DataLoader::await();
     }
 
-    protected function onWait(Promise $promise)
+    protected function onWait(Promise $promise): void
     {
         DataLoader::await();
     }