[Plugin: Multiple Post Thumbnails] array add class not working

This seemed to do the trick:

if (class_exists('MultiPostThumbnails')) {
          $image_name = "sidebar-image";
                if (MultiPostThumbnails::has_post_thumbnail('page', $image_name)) {
                    $image_id = MultiPostThumbnails::get_post_thumbnail_id( 'page', $image_name, $post->ID );
                    $attr = array('class' => "border-1");
                    $image = wp_get_attachment_image( $image_id, 'sidebar-image', false, $attr );
                    echo $image;
                }
        };