Index: includes/menu.inc
===================================================================
RCS file: /cvs/drupal/drupal/includes/menu.inc,v
retrieving revision 1.306
diff -u -p -r1.306 menu.inc
--- includes/menu.inc	15 Nov 2008 11:45:03 -0000	1.306
+++ includes/menu.inc	23 Nov 2008 01:03:21 -0000
@@ -500,7 +500,12 @@ function _menu_check_access(&$item, $map
       $item['access'] = (count($arguments) == 1) ? user_access($arguments[0]) : user_access($arguments[0], $arguments[1]);
     }
     else {
-      $item['access'] = call_user_func_array($callback, $arguments);
+      if (drupal_function_exists($callback)) {
+        $item['access'] = call_user_func_array($callback, $arguments);
+      }
+      else {
+        $item['access'] = FALSE;
+      }
     }
   }
 }