diff --git includes/form.inc includes/form.inc
index 4dd0626..6dee7bb 100644
--- includes/form.inc
+++ includes/form.inc
@@ -1346,6 +1346,10 @@ function form_state_values_clean(&$form_state) {
       $values = &$form_state['values'];
       $last_parent = array_pop($parents);
       foreach ($parents as $parent) {
+        if (!isset($values[$parent])) {
+          // There is no value for this button, so bail out.
+          continue 2;
+        }
         $values = &$values[$parent];
       }
       unset($values[$last_parent]);