Query on Firebase Model Error

[goddamnyouryan]

I am trying to do a query on a firebase model (like it says we can do in the docs) and I am getting a strange error I don't quite understand. Here is my model (in coffeescript):

class Entities.InvoiceCharge extends Backbone.Firebase.Model
  firebase: "< MY FIREBASE >/charges"
  url: ->
    new Firebase(@firebase).orderByChild('invoice').equalTo('-JsJtr4kpzJtJY9lJKbB')

Just wanting to return the first "charge" that has an "invoice" attribute equal to '-JsJtr4kpzJtJY9lJKbB' but I am getting a strange error:

Uncaught TypeError: this.firebase.update is not a function

Which is referencing this line:

  // apply local changes remotely
  this._listenLocalChange(function(model) {
    this.firebase.update(model);
  });

Any idea what might be causing this? I've inspected this firebase instance, it looks normal to me but for some reason doesn't have the update function attached to it.