Drupal shows "node_form_validate() undefined" error while trying to create a poll

I have a Drupal 7 website and would like to add a poll to it. I have enabled a Poll module and opened Add material -> Poll page. When I pressed "add variant" button I got an error stating that

AJAX HTTP error.
HTTP response code: 200
Debugging information follows.
Path: /sy开发者_StackOverflow中文版stem/ajax
Text status: OK
ResponseText: 
Fatal error:  Call to undefined function node_form_validate() in /*/public_html/includes/form.inc on line 1390

How can it be fixed? I have rather small experience with Drupal and hardly imagine where to start.

Hmmm... Seems I have fixed it myself by adding

require_once(DRUPAL_ROOT . "/modules/node/node.pages.inc");

to /modules/poll/poll.module as described here. It turned out this Drupal is not as a terrible CMS.