diff --git a/lib/Doctrine/ODM/MongoDB/Persisters/CollectionPersister.php b/lib/Doctrine/ODM/MongoDB/Persisters/CollectionPersister.php
index d12696562a..6df2fce834 100644
--- a/lib/Doctrine/ODM/MongoDB/Persisters/CollectionPersister.php
+++ b/lib/Doctrine/ODM/MongoDB/Persisters/CollectionPersister.php
@@ -284,7 +284,7 @@ private function executeQuery(object $document, array $newObj, array $options) :
         }
     }
 
-    private function excludeSubPaths(array $paths)
+    private function excludeSubPaths(array $paths) : array
     {
         $checkedPaths = [];
         $pathsAmount  = count($paths);