sorting objects in Ember, Ember.SortableMixin sortProperties not working as expected

I'm new to Ember and recently followed the Ember "getting started" guide to build TodoMVC. Here it is in a jsbin.

Everything works, but now I'm trying to sort the todos by adding sortProperties to the TodosController:

Todos.TodosController = Ember.ArrayController.extend({
    sortProperties: ['title'],
    sortAscending: true,
    ...
});

And in the template, I have this:

{{#each todo in arrangedContent itemController="todo"}}
  ...
{{/each}}

Based on the Ember.SortableMixin documentation, what I'm doing seems to be reasonable but clearly I'm missing something.

Any help would be much appreciated!