diff --git a/src/Rebing/GraphQL/GraphQL.php b/src/Rebing/GraphQL/GraphQL.php
index <HASH>..<HASH> 100644
--- a/src/Rebing/GraphQL/GraphQL.php
+++ b/src/Rebing/GraphQL/GraphQL.php
@@ -304,7 +304,7 @@ class GraphQL
         return $this->schemas;
     }
 
-    protected function clearTypeInstances()
+    protected function clearTypeInstances(): void
     {
         $this->typesInstances = [];
     }
Add types to \Rebing\GraphQL\GraphQL::clearTypeInstances