XF 2.0 Using mover to move a thread and set prefix, what have I missed?

[Marcel]

$mover = \XF::app()->service('XF:Thread\Mover', $thread);
if ($forum->ut_archive_prefix_id)
{
    $mover->setPrefix($forum->ut_archive_prefix_id);
}
else if ($destinationForum->default_prefix_id)
{
    $mover->setPrefix($destinationForum->default_prefix_id);
}
$mover->move($destinationForum);

Pseudo : If current forum has ut_archive_prefix_id set...then use that
If not, then check if the destination has a default prefix_id. If so, use that.
If not...then leave as is and move the thread.

The relevant fields are all set, it just moves the thread with no prefix at all.

 

[Marcel]

Oh bumnuggets. My bad!
The prefix set in $forum->ut_archive_prefix_id must be available to the destination forum or else it just strips it out.