diff --git a/Classes/Domain/Model/Document.php b/Classes/Domain/Model/Document.php
index 4ef462016a..7e3f430fde 100644
--- a/Classes/Domain/Model/Document.php
+++ b/Classes/Domain/Model/Document.php
@@ -638,7 +638,7 @@ public function addCollection(Collection $collection): void
      *
      * @param Collection $collection
      */
-    public function removeCollection(Collection $collection)
+    public function removeCollection(Collection $collection): void
     {
         $this->collections->detach($collection);
     }