Nested index route not rendering in outlet properly

I'm trying to render taking-cash/index inside taking-cash's :

router.js:

  this.route('retirement-options', function () {
    this.route('taking-cash');

It doesn't render it inside the outlet, unless I explicitly specify the /index path:

  this.route('retirement-options', function () {
    this.route('taking-cash', function() {
      this.route('index', { path: '' });
    });
  });

Why isn't /index implied, can I not specify it in router.js?