Sorting on numerical model attribute in Ember

I've got a very basic Price model that looks like this:

App.Price = DS.Model.extend({
    value: DS.attr()
});

App.Price.reopenClass({
    FIXTURES: [
        { id: 1, value: 29.99 },
        { id: 2, value: 39.99 },
        { id: 3, value: 49.99 },
        { id: 4, value: 55.99 }
    ]
});

Here's the route that's using this model:

App.PricingRoute = Ember.Route.extend({
    model: function(){
        return this.store.find('price');
    }
});

In the controller, I set the sorting to be based on the value attribute:

App.PricingController = Ember.ArrayController.extend({
    sortProperties: ['value'],
    sortAscending: true
});

Then my template (in Jade) where I want them to be display in sorted order:

{{#each price in this}}
    li.price-levels__value {{price.value}}
        .price-levels__remove("{{action 'delete' price.id}}")
{{/each}}

Problem is they're not sorted. Interesting fact is that if I change the type of the value attribute to strings, the sorting DOES work.

eg.

{ id: 1, value: '29.99' }
{ id: 2, value: '39.99' }
etc.

So how do I get the sorting to work on a numerical model attribute?