diff U3 C:/temp/adress/uc_addresses.module_old C:/temp/adress/uc_addresses.module
--- C:/temp/adress/uc_addresses.module_old	Wed Mar 17 18:05:28 2010
+++ C:/temp/adress/uc_addresses.module	Tue Sep 21 16:50:00 2010
@@ -959,7 +959,11 @@
     $valid = TRUE;
     foreach (element_children($form_state['values']['panes']) as $pane_id) {
       $func = _address_pane_data($pane_id, 'callback');
-      $isvalid = $func('process', $address, $form_state['values']['panes'][$pane_id]);
+
+	  if(!empty($func) AND function_exists($func)) {
+        $isvalid = $func('process', $address, $form_state['values']['panes'][$pane_id]);
+	  }
+
       if ($isvalid === FALSE) {
 	$_SESSION['expanded_panes'][] = $key;
 	$valid = FALSE;