[Plugin: WordPress.com Popular Posts] Category ID not working

The following will not display any posts.

$cat = get_query_var('cat');

WPPP_show_popular_posts( 'title=&number=20&days=90&show=posts&format=<a href="%post_permalink%" title="%post_title_attribute%">%post_title%</a>&category='.$cat.'' );

The only way I can get it to work is to do category=0. Even doing category=41 does not work (assuming it’s an active category).

http://ww.wp.xz.cn/extend/plugins/wordpresscom-popular-posts/