Ember Data: saving polymorphic relationships

I'm having trouble saving "hasMany" polymorphic records in Ember Data (1.0.0-beta.15). It looks as if Ember Data isn't setting the "type" property of the polymorphic relationship. Relationships in serialized records look like:

"roles": ["1", "2"]

When I expect them to look more like:

"roles":[{
    "id": "1",
    "type": "professionalRole"
  }, {
    "id": "2",
    "type": "personalRole"
  }
];

I see the following error in the console:

TypeError: Cannot read property 'typeKey' of undefined

If the records come back from the server in the expected format, all is well. The error only occurs when Ember Data creates the relationship.

I experience this using the FixtureAdapter, LocalStorageAdapter, and the RESTAdapter. I've read every piece of documentation I can find on the subject, but I cannot see my mistake.

I've created a CodePen to demonstrate the problem, but I'll also paste that code below.

window.App = Ember.Application.create();
App.ApplicationAdapter = DS.FixtureAdapter;

App.Person = DS.Model.extend({
  name: DS.attr(),
  roles: DS.hasMany('role')
});

App.Role = DS.Model.extend({
  title: DS.attr(),
  person: DS.belongsTo('person', {
    polymorphic: true
  })
});

App.ProfessionalRole = App.Role.extend({
  rank: DS.attr()
});

App.ApplicationRoute = Ember.Route.extend({
  setupController: function(controller) {
    var person = this.store.createRecord('person', {
      name: 'James'
    });
    var role = this.store.createRecord('professionalRole', {
      title: 'Code Reviewer',
      rank: 'Expert'
    });
    var promises = Ember.RSVP.hash({
      person: person.save(),
      role: role.save()
    });
    promises.catch(function() {
      controller.set('initialSaveResult', 'Failure');
    });
    promises.then(function(resolved) {
      controller.set('initialSaveResult', 'Success!');
      var resolvedPerson = resolved.person;
      var resolvedRole = resolved.role;

      // Either/both of these break it
      //resolvedRole.set('person', resolvedPerson);
      resolvedPerson.get('roles').addObject(resolvedRole);

      var innerPromises = Ember.RSVP.hash({
        person: resolvedPerson.save(),
        role: resolvedRole.save()
      });
      innerPromises.catch(function() {
        controller.set('secondSaveResult', 'Failure');
      });
      innerPromises.then(function() {
        controller.set('secondSaveResult', 'Success!');
      });
    });
  }
});

App.ApplicationController = Ember.Controller.extend({
  initialSaveResult: "Loading...",
  secondSaveResult: "Loading..."
});