Fatal Error when getting Messages

[ehoffman73]

Fatal error: Unsupported operand types in /home/sitename/public_html/includes/functions_user.php on line 251

Do we know what can cause this, when its very random users that are getting this? I don't immediately see an answer searching around...is there even a good thing to troubleshoot?

--------------- Added [DATE]1315709539[/DATE] at [TIME]1315709539[/TIME] ---------------

This is the code at that point:

if (!empty($vbulletin->userinfo['pmfolders']))
	{
		$pmfolders = $pmfolders + unserialize($vbulletin->userinfo['pmfolders']);
	}