Error when trying to annex a file with a client user

If i try to annex a file with a user from a client company i get this error:

[18-Jan-2009 12:20:00] PHP Fatal error: Using $this when not in object context in ~/Sites/projectpier/application/models/project_files/ProjectFiles.class.php on line 154

Apparently the problem is with:
$conditions = array('`project_id` =? AND `folder_id` = ? AND `is_private` = ?', $project->getId(), $this->getId(), false);

I replaced it with:
$conditions = array('`project_id` =? AND `folder_id` = ? AND `is_private` = ?', $project->getId(), $folder->getId(), false);

And now it works fine.

PS: Annexing files with the admin user works fine.