custom post types in category page

i made a custom post type (cover). want i wanted to do is to show only the custom post when viewing a category.

i tried using post queries within the loop. the shortest one i tried is this:

<?php query_posts( ‘post_type=cover’); ?>

however, what happens is that, instead of showing only the custom post in the particular category being viewed, it shows ALL custom posts.

how can i fix this?