[Avatars] Adding class to Avatar

I'm trying to add a class "rounded-circle" to the avatar. With WordPress' standard avatars (gravitar) the following code works, but not when using Avatars plugin.

`get_avatar(get_current_user_id(), '60', null, null, array( 'class' => array( 'rounded-circle' ) ) );`

How can I add a class when using Avatars plugin?