--- includes/form.inc
+++ includes/form.inc
@@ -894,7 +894,7 @@
   // Recurse through all child elements.
   $count = 0;
   foreach (element_children($form) as $key) {
-    $form[$key]['#post'] = $form['#post'];
+    $form[$key]['#post'] = isset($form['#post']) ? $form['#post'] : array();
     $form[$key]['#programmed'] = $form['#programmed'];
     // Don't squash an existing tree value.
     if (!isset($form[$key]['#tree'])) {