--- ec_checkout.checkout.inc	2011-04-27 18:11:17.000000000 +0200
+++ ec_checkout.checkout.inc	2011-04-27 18:11:20.000000000 +0200
@@ -147,6 +147,8 @@ function ec_checkout_checkout_form(&$for
  * Implementation of hook_checkout_update().
  */
 function ec_checkout_checkout_update(&$form, &$form_state) {
+  if (!isset($form_state['values']['items'])) return;
+
   $txn = & $form_state['txn'];
 
   foreach ($form_state['values']['items'] as $nid => $item) {