I made use of the next codes within the function.php to produce a custom feed.feed-cust.php (copy of feed-rss2.php) was put in the theme folder. However the url example.com/?feed=feedname throws the mistake..Make sure you suggest to repair the mistake.

ERROR: feedname isn't a valid feed template.

function my_feed() {
    load_template( TEMPLATEPATH . '/feed-cust.php');
    }
    add_feed('feedname', 'my_feed');