I try to add custom class on get avatar in comment walker

get_avatar( $comment, $args['avatar_size'], '', '', array('class' => 'myclass') );

But class doesn’t show in source code. Thx

Edit : Output

<img src="...default.png" width="48" height="48" alt="Avatar" class="avatar avatar-48wp-user-avatar wp-user-avatar-48 alignnone photo avatar-default">