Why does admin_body_class not work?

This works for the admin backend; I needed to restructure the filter to be able to add a priority:

function add_admin_body_class($classes) {
$user = wp_get_current_user();
foreach ($user->roles as $user_role) {
    $classes .= " role-{$user_role}";
  }
  return $classes;
}
add_filter("admin_body_class", "add_admin_body_class", 9999);