theme_nice_menus_main_menu() / ...secondary_menu(): $direction is marked 'optional' but it's not

The Doxygen comment indicates that the $direction parameter is optional. It's not: without passing it I get a "Notice: Undefined index: direction" warning.

See nice_menus.module:530:

/**
 * Theme the main menu as a Nice menu.
 *
 * @param $direction
 *   Optional. The direction the menu expands. Default is 'down'.
[...]
*/
function theme_nice_menus_main_menu($variables) {
  $direction = $variables['direction'];
[...]
}

The same applies for theme_nice_menus_secondary_menu().