diff --git a/Mapping/Converter.php b/Mapping/Converter.php
index 6778c0f6..2d265f93 100644
--- a/Mapping/Converter.php
+++ b/Mapping/Converter.php
@@ -47,6 +47,10 @@ public function convertDocumentToArray($document): array
 
     protected function normalize($document, $metadata = null)
     {
+        if ($document === null) {
+            return null;
+        }
+
         $metadata = $metadata ?? $this->propertyMetadata[get_class($document)];
         $result = [];