Ember computed properties on array?

Having trouble with computed properties on an array - they don’t seem to update. I’m trying to model a queue backed as an Ember.Enumerable backed by a regular array. See

Ember Twiddle

When the queue is first created, it’s length property, which is a computed property on the items array, is undefined. When I add items to my queue, the length computed property goes to 1, 2, 3 etc as I’d expect and when I clear the queue, it goes back to zero. But when the queue is first constructed, the length is undefined for some strange reason, even though all the code is properly using Ember objects, get, set and pushObject, etc.

Any ideas?