"Missing argument 3 for ajax_validator()" watchdog php error

Have noticed that the ajax module seems to fire a php error every time it functions. Watchdog logs:

Type php
Date Wednesday, April 22, 2009 - 16:08
User magjor
Location http://localhost/envelopes_online_draft1/transactionform
Referrer http://localhost/envelopes_online_draft1/transactionform
Message Missing argument 3 for ajax_validator(), called in C:\xampp\htdocs\envelopes_online_draft1\includes\form.inc on line 766 and defined in C:\xampp\htdocs\envelopes_online_draft1\sites\all\modules\drupal-contrib\ajax\ajax.module on line 390.
Severity error
Hostname 127.0.0.1
Operations 

Created a very simple test form on a clean drupal 6.10 installation, and the error was still logged. See http://drupal.org/node/441316#comment-1514256 (turns out it had nothing to do with the select field, as I removed that, and it still fired). Noticed the issue in dev as well as 6.13.