Why do I need to define an index route in Ember.js?

Previously, my routes were defined:

this.route('username', {
  path: '/:username'
}, function() {
  this.route("room", {
    path: "/:room",
});

I changed it to:

this.route('username', {
    path: '/:username'
  }, function() {
    this.route('index')
});
this.route("room", {
  path: "/:room"
});

This works fine. But my username/index route will not load unless I explicitly define index in router.js.

The following will not work:

this.route('username', {
  path: '/:username'
});
this.route("room", {
  path: "/:room",
});

Is this normal behavior?