--- webform.module.org	2010-06-21 06:40:21.000000000 +0200
+++ webform.module	2010-07-27 12:01:10.000000000 +0200
@@ -1717,7 +1717,7 @@ function _webform_client_form_validate($
             }
           }
         }
-        elseif (!isset($options[$elements['#value']])) {
+        elseif ($elements['#value'] && !isset($options[$elements['#value']])) {
           form_error($elements, t('An illegal choice has been detected. Please contact the site administrator.'));
           watchdog('form', 'Illegal choice %choice in %name element.', array('%choice' => $elements['#value'], '%name' => empty($elements['#title']) ? $elements['#parents'][0] : $elements['#title']), WATCHDOG_ERROR);
         }