Index: captcha.module
===================================================================
--- captcha.module	(revision 1109)
+++ captcha.module	(revision 1110)
@@ -495,7 +503,9 @@
  *     if the values could not be found, e.g. for a fresh form).
  */
 function _captcha_get_posted_captcha_info($element, $form_state, $this_form_id) {
-  if ($form_state['submitted'] && isset($form_state['captcha_info'])) {
+  // If the form is submited via AJAX, $form_state['submitted'] will be FALSE,
+  // check $form_state['triggering_element'] instead
+  if (isset($form_state['triggering_element']['#value']) && isset($form_state['captcha_info'])) {
     // We are handling (or rebuilding) an already submitted form,
     // so we already determined the posted form ID and CAPTCHA session ID
     // for this form (from before submitting). Reuse this info.