Remove Single Category from RSS Feed

[seth17]

What's the best way to remove a single category from the RSS feed? I tried this code but it doesn't seem to work. Is there another or better way to filter it out?

PHP Code:

add_filter('pre_get_posts','custom_feed_filter');
function custom_feed_filter($query) {
    if( $query->is_feed )
        $query->set('cat','-11');
    return $query;
}