diff --git a/lib/Doctrine/ORM/Tools/SchemaTool.php b/lib/Doctrine/ORM/Tools/SchemaTool.php
index 571671be3..1c7359354 100644
--- a/lib/Doctrine/ORM/Tools/SchemaTool.php
+++ b/lib/Doctrine/ORM/Tools/SchemaTool.php
@@ -424,7 +424,7 @@ class SchemaTool
             }
 
             if (isset($joinColumn['unique']) && $joinColumn['unique'] == true) {
-                $uniqueConstraints[] = array('columns' => $columnName);
+                $uniqueConstraints[] = array('columns' => array($columnName));
             }
 
             if (isset($joinColumn['onUpdate'])) {