diff --git a/includes/form.inc b/includes/form.inc
index c3e5954..04cee25 100644
--- a/includes/form.inc
+++ b/includes/form.inc
@@ -2154,7 +2154,9 @@ function form_state_values_clean(&$form_state) {
     foreach ($parents as $parent) {
       $values = &$values[$parent];
     }
-    unset($values[$last_parent]);
+    if (is_array($values)) {
+      unset($values[$last_parent]);
+    }
   }
 }