$in_active_trail variable in menu-item.tpl.php

I'm creating a theme in Drupal 6.1.

When I create a file named menu-item.tpl.php in my theme, I can't seem to get the variable $in_active_trail to work.
However, If I make a similar function (phptemplate_menu_item) in my template.php everything works as expected.

Am I missing something?