Call to undefined function privatemsg_list_submit()

Hey, just cross-posting out of desperation. I've implemented thread view in a D7 Panel and it's working well to display the various messages. There's a major problem with the actions on the form though - whenever any actions (mark, delete) are selected, I get a javascript alert:

An AJAX HTTP error occurred. 
HTTP Result Code: 200
Debugging information follows.
Path: /system/ajax
StatusText: OK
ResponseText: Fatal error: Call to undefined function privatemsg_list_submit() in [...]/includes/form.inc on line 1464

This panel/view is on the theme side and I get the feeling that there's a file that isn't being included in the form render or something. The other thread is here: http://drupal.org/node/1961776. Any chance you have an idea of how to fix this? Thanks for your help...