How to setup gitweb with categories

Hello,

I'm trying to setup a gitweb frontend with categorized projects. I already 

have a plain project list running an changed my gitweb.conf so that it 
contains

our $projects_list_group_categories = "1";
our $project_list_default_category = "foo";

I'd now expect all projects to be shown under the category "foo", but upon 

reload nothing happens on the web page.

Can anyone tell me what's wrong?

Best regards

Frank