Custom Post Types and Shortcodes

The shortcode for the most/highest/lowest rated posts works with custom post types when i do this:

<?php get_highest_rated('my-post-type', 0, 10); ?>

BUT when i try to use it to get the most/highest/lowest rated posts in a given time period with a custom post type it doesn’t work…

<?php get_highest_rated_range('1 month', 'my-post-type', 10); ?>

Is there a problem with the plugin or am i doing something wrong?