I have a custom post type called "artist" with a custom

I have a custom post type called "artist" with a custom meta field for artists last name. I would like the archive page to display the artists alphabetically by last name.

This is what I have now:

<?php

$args = array(
      	'post_type' => 'artist',
	'orderby' => 'meta_value',
	'meta_key' => 'artist_meta_artist_last_name',
	'order' => 'ASC',
	'posts_per_page' => -1
   );
   $category_posts = new WP_Query($args);

   if($category_posts->have_posts()) :
      while($category_posts->have_posts()) :
         $category_posts->the_post();
?>

If I change it from ASC to DSSC it changes the order, but I'm not sure what its ordering by. Certainly not the meta_key and not by date or title. I have looked everywhere for a solution, but none have worked.