default image broken with theme function

I'm having a similar problem to the one described here, except for 6 instead of 5. Basically I have a user relationship block tpl file that references an imagecache preset "user_thumb" using the following code-

<?php
theme('imagecache', 'user_thumb', $relation_to->picture)
?>

It works fine except that it doesn't load the default picture for users without custom pictures uploaded. The rendered placeholder does not even reference a missing image file, it just references the imagecache preset folder, in my case "user_thumb/".

Any help would be greatly appreciated!

Keith