Index: includes/file.inc
===================================================================
RCS file: /cvs/drupal/drupal/includes/file.inc,v
retrieving revision 1.174
diff -u -r1.174 file.inc
--- includes/file.inc	10 Jul 2009 04:08:20 -0000	1.174
+++ includes/file.inc	15 Jul 2009 04:44:38 -0000
@@ -1043,8 +1043,10 @@
   // Call the validation functions specified by this function's caller.
   $errors = array();
   foreach ($validators as $function => $args) {
-    array_unshift($args, $file);
-    $errors = array_merge($errors, call_user_func_array($function, $args));
+    if (drupal_function_exists($function)) {
+      array_unshift($args, $file);
+      $errors = array_merge($errors, call_user_func_array($function, $args));
+    }
   }
 
   // Let other modules perform validation on the new file.