link-to route with an optional dynamic segment is undefined

I have an "optional" dynamic segment on the index route.

Navigating to the index (http://foo.com) loads the default template, index.hbs.

Navigating to a dynamic segment on the index (http://foo.com/123) loads another template, chat.hbs.

router.js

Router.map(function() {
this.route('index', { path: '/' });
this.route('index', { path: '/:room_id' });

routes/index.js

model(params) {
  if (params.room_id) {
    this.set('templateName', 'chat');
    return params.room_id;
  }
}

All is ok until using the link-to helper. Home

This generates a link to http://ift.tt/2cuVxRA instead of just http://foo.com

I have tried passing in a model to the link-to helper, but have not found a way to make it work.