Index: modules/captcha/captcha.module
===================================================================
--- modules/captcha/captcha.module	(revision 55155)
+++ modules/captcha/captcha.module	(revision )
@@ -210,6 +210,9 @@
     '#value' => $captcha_sid,
   );
 
+  // Make sure the $form_state['values']['captcha_response'] exists if the form has #tree=true
+  $element['#tree'] = FALSE;
+
   // Additional one time CAPTCHA token: store in database and send with form.
   $captcha_token = md5(mt_rand());
   db_query("UPDATE {captcha_sessions} SET token='%s' WHERE csid=%d", $captcha_token, $captcha_sid);