diff --git a/core/modules/system/system.install b/core/modules/system/system.install
index 06935d1..4e3082c 100644
--- a/core/modules/system/system.install
+++ b/core/modules/system/system.install
@@ -1460,6 +1460,7 @@ function system_schema() {
       ),
     ),
     'primary key' => array('pid'),
+    'unique keys' => array('alias_langcode' => array('alias', 'langcode')),
     'indexes' => array(
       'alias_langcode_pid' => array('alias', 'langcode', 'pid'),
       'source_langcode_pid' => array('source', 'langcode', 'pid'),