=== modified file 'includes/menu.inc'
--- includes/menu.inc	2008-05-13 17:38:42 +0000
+++ includes/menu.inc	2008-05-21 16:26:54 +0000
@@ -205,6 +205,10 @@ function menu_get_ancestors($parts) {
   $length =  $number_parts - 1;
   $end = (1 << $number_parts) - 1;
   $masks = variable_get('menu_masks', array());
+  if (!$masks) {
+    menu_rebuild();
+    $masks = variable_get('menu_masks', array());
+  }
   // Only examine patterns that actually exist as router items (the masks).
   foreach ($masks as $i) {
     if ($i > $end) {