[Plugin: WordPress SEO by Yoast] %Êtegory_description%% not showing up in meta description

Hello,

%%category_description%% tag seems not to be working, at least in the meta description field where I’ve tested it, the category description is not showing up when I look at the source code.

Is this a known bug?

Thanks.

http://wordpress.org/extend/plugins/wordpress-seo/