Index: sites/all/modules/captcha/captcha.module
===================================================================
--- sites/all/modules/captcha/captcha.module  (revision 4325)
+++ sites/all/modules/captcha/captcha.module  (revision 4327)
@@ -458,7 +458,7 @@
   }
   // Return the posted form_id and CAPTCHA session ID.
   $posted_form_id = isset($post_data['form_id']) ? $post_data['form_id'] : NULL;
-  $posted_captcha_sid = isset($post_data['captcha_sid']) ? $post_data['captcha_sid'] : NULL;
+  $posted_captcha_sid = isset($post_data['captcha_sid']) ? intval($post_data['captcha_sid']) : NULL;
   return array($posted_form_id, $posted_captcha_sid);
 }