diff --git a/src/DoctrineWriter.php b/src/DoctrineWriter.php
index a481ad6..9673119 100644
--- a/src/DoctrineWriter.php
+++ b/src/DoctrineWriter.php
@@ -99,6 +99,8 @@ public function __construct(
             } else {
                 $this->lookupFields = [$index];
             }
+        } else {
+            $this->lookupFields = $this->objectMetadata->getIdentifierFieldNames();
         }
 
         if (!method_exists($this->objectRepository, $lookupMethod)) {