Help with setting / route

In a multi module application I'm trying to set the / route using:

  $router->add('/', array(
  'module' => 'frontend',
  'namespace' => 'Ammo\Frontend\Modules\Application\Controllers',
  'controller' => 'index',
  'action' => 'someaction'
  ));
$router->notFound(array(
        'module' => 'frontend',
        'namespace' => 'Ammo\Frontend\Modules\Application\Controllers',
        "controller" => "error",
        "action" => "route404"
));

the router seems to ignore this entry for the web site home (no route). I've tried with:

$router = new Router(FALSE); and $router = new Router();

web site root in always fires NotFound route.

Any help??