add_feed not working

I hope to add a ‘select’ feed, but the following plugin code does not add it. …/feed/select/ is not found. If I use add_feed(‘rss’,’feed_select’), the …/feed/rss/ returns the normal rss feed.

function feed_select() {
	echo('test successful');
}
function add_select() {
	add_feed('select','feed_select');
}
add_action('init','add_select');