diff --git a/Validator/Constraint/UniqueDocumentValidator.php b/Validator/Constraint/UniqueDocumentValidator.php
index 9b77a3e..4aca6c0 100644
--- a/Validator/Constraint/UniqueDocumentValidator.php
+++ b/Validator/Constraint/UniqueDocumentValidator.php
@@ -116,6 +116,9 @@ private function createCriteria(Document $document, array $fields, array $caseIn
         $criteria = array();
         foreach ($fields as $field) {
             $value = $document->get($field);
+            if ($value instanceof Document) {
+                $value = $value->getId();
+            }
             if (in_array($field, $caseInsensitive)) {
                 $value = new \MongoRegex(sprintf('/^%s$/i', $value));
             }