Fork-created model not listening to Firebase

[neverfox]

Route:

PortfolioAddAllocationRoute = Ember.Route.extend
  beforeModel: ->
    @set "fork", @store.fork()

  model: (params) ->
    @fork.fetch("portfolio", @modelFor("portfolio").get "id").then (portfolioFork) =>
      @set "allocation", portfolioFork.get("allocations").pushObject @fork.createRecord "allocation"
      @get "allocation"

  deactivate: ->
    @fork.destroy()

  actions:
    save: ->
      @get("allocation").save().then =>
        console.log "Saved"
        @transitionTo "portfolio"
    cancel: ->
      @transitionTo "portfolio"

Later, on the controller for this route:

actions:
  addPosition: ->
    position = @get("positions").pushObject @get("fork").createRecord "position"

This route is designed to let the user add a new allocation, and when they're done adding positions to it, save it or cancel. However, when I transition after the save, none of the position models (embedded) associated with this new allocation are listening to Firebase (though everything saved just fine). And if I try to view the data for allocation models in Ember Inspector, I get a Uncaught Error: Firebase.child failed: First argument was an invalid path: "undefined". Paths must be non-empty strings and can't contain ".", "#", "$", "[", or "]" from firebase.js. If I refresh the page, everything syncs ok. If I don't use a fork, and instead use the regular store, everything works fine (but I lose the auto-self-destruct of the cancel functionality, of course). Is it obvious from this what might be going wrong?