Contributor page by author last name

I’m trying to list my authors by last name, and have copied the section I need from twentyfourteen functions.php

And edited just the following section from post count to last name but it doesn’t seem to be working. The first post is correct, then they all go completely random.

function twentyfourteen_list_authors() {
	$contributor_ids = get_users( array(
		'fields'  => 'ID',
		'orderby' => 'last_name',
		'order'   => 'DESC',
		'who'     => 'authors',
	) );

Am I missing something?

Thanks!