Index: wp-includes/query.php
===================================================================
--- wp-includes/query.php	(revision 21755)
+++ wp-includes/query.php	(working copy)
@@ -1398,6 +1398,7 @@
 			, 's'
 			, 'sentence'
 			, 'fields'
+			, 'menu_order'
 		);
 
 		foreach ( $keys as $key ) {