--- includes/form.inc	2008-10-16 05:43:08.000000000 -0700
+++ includes/form-fixed.inc	2008-11-16 17:44:35.000000000 -0800
@@ -701,7 +701,7 @@ function _form_validate($elements, &$for
             }
           }
         }
-        elseif (!isset($options[$elements['#value']])) {
+        elseif (!empty($elements['#value']) && !isset($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);
         }