diff --git a/lib/Doctrine/ODM/MongoDB/Aggregation/Aggregation.php b/lib/Doctrine/ODM/MongoDB/Aggregation/Aggregation.php
index 603eb09b9..eb860a58c 100644
--- a/lib/Doctrine/ODM/MongoDB/Aggregation/Aggregation.php
+++ b/lib/Doctrine/ODM/MongoDB/Aggregation/Aggregation.php
@@ -59,7 +59,7 @@ public function getIterator(): Iterator
         return $this->prepareIterator($cursor);
     }
 
-    private function prepareIterator(CursorInterface $cursor): Iterator
+    private function prepareIterator(CursorInterface&Iterator $cursor): Iterator
     {
         if ($this->classMetadata) {
             $cursor = new HydratingIterator($cursor, $this->dm->getUnitOfWork(), $this->classMetadata);