TodoMVC with ember, id does not increment

I am following the getting started guide from emberjs, and am at the point where I can add todos. My problem is though that when I add a todo it has an id value of null - is there a practical way to auto increment this?

var TodosController = Ember.ArrayController.extend({
actions: {
    createTodo: function() {
        var title = this.get('newTitle');
        if (!title.trim()) {
            return;
        }

        var todo = this.store.createRecord('todo', {
            title: title,
            isCompleted: false
        });

        this.set('newTitle', '');

        todo.save();
    }
}

});