Adding custom class to get_avatar() image doesn&https://wordpress.stackexchange.com/https://wordpress.stackexchange.com/#39;t work

The following worked for me:

<?php echo get_avatar( $comment, 60, '', '', $args = array( 'scheme' => 'https', 'class' => 'myclass' ) ); ?>

Your use of $args['avatar_size'] should be an int and you may have confused the use of this parameter (unless you have a variable $args, and it is an array).