JOIN Query Not Working

This works:

$authors = $wpdb->get_results("
SELECT DISTINCT u.ID, u.user_nicename
FROM      $wpdb->users u JOIN $wpdb->posts p ON p.post_author = u.ID
WHERE     (p.post_status = 'publish') AND (p.post_type = 'post')
ORDER BY user_nicename
");

This does not:

$authors = $wpdb->get_results("
SELECT DISTINCT u.ID, u.user_nicename, um.meta_value AS last_name
FROM $wpdb->users u JOIN $wpdb->posts p ON p.post_author = u.ID
JOIN $wpdb->usermeta um ON (um.user_id=u.ID AND meta_key='last_name')
WHERE (p.post_status = 'publish') AND (p.post_type = 'post')
ORDER BY last_name
");

The second query works fine in SQL Manager.

Thanks for any tips.