diff --git a/src/Adapter/MapperInterface.php b/src/Adapter/MapperInterface.php
index <HASH>..<HASH> 100644
--- a/src/Adapter/MapperInterface.php
+++ b/src/Adapter/MapperInterface.php
@@ -33,5 +33,5 @@ interface MapperInterface
      *
      * @return void
      */
-    public function map(array $input, MetadataInterface $output);
+    public function map(array $input, MetadataInterface &$output);
 }
Bugfix: Mapper should use by-reference