How to debug a failing Ember Data JSON API save() operation?

I'm creating an Ember Data object with this action in a route:

    createProfile: function() {
        var profile = this.store.createRecord('profile', {
            username: 'bob',
            orientation: 'straight',
            gender: 'male',
            firstname: 'kevin',
            lastlogin: 'today'
        });
        profile.save().then(function() {
            this.transitionTo('welcome');
        });
    }

On the server side I've handling the POST request and returning a response that looks like this:

Request URL: http://ift.tt/1QYtmr7
Request Method: POST
Status Code: 201 Created  <-- It has got the "Green Dot" in Ember Inspector

Response headers:
Access-Control-Allow-Headers: x-requested-with, Content-Type, origin, authorization, accept, client-security-token
Access-Control-Allow-Methods: POST, GET, OPTIONS, DELETE, PUT
Access-Control-Allow-Origin: *
Access-Control-Max-Age: 1000
Connection: Keep-Alive
Content-Length: 164
Content-Type: application/vnd.api+json; charset=UTF-8
Date: Wed, 07 Oct 2015 07:17:06 GMT
Keep-Alive: timeout=5, max=100
Location: http://localhost/api/1/profiles/2
Server: waitress
access-control-allow-credentials: true

And the body of the response looks like this:

{'data': {'type': 'profiles', 'attributes': {'firstname': 'kevin', 'lastlogin': 'today', 'username': 'bob', 'orientation': 'straight', 'gender': 'male'}, 'id': '2'}}

As far as I can tell, this all looks good. I've done another operation that successfully does a GET method on this model type. I can see the model appear in the Ember Inspector under the Ember Data panel and see it is correctly given an Id of 2.

But then when the server returns the response the error message is:

createProfile/<@http://localhost/assets/emberpd.js:324:6
tryCatch@http://localhost/assets/vendor.js:60951:14
invokeCallback@http://localhost/assets/vendor.js:60966:15
publish@http://localhost/assets/vendor.js:60934:9
@http://localhost/assets/vendor.js:40181:7
Queue.prototype.invoke@http://localhost/assets/vendor.js:11532:9
Queue.prototype.flush@http://localhost/assets/vendor.js:11596:11
DeferredActionQueues.prototype.flush@http://localhost/assets/vendor.js:11392:11
Backburner.prototype.end@http://localhost/assets/vendor.js:10720:9
Backburner.prototype.run@http://localhost/assets/vendor.js:10842:13
run@http://localhost/assets/vendor.js:29679:12
ember$data$lib$adapters$rest$adapter$$RESTAdapter<.ajax/</hash.success@http://localhost/assets/vendor.js:66323:15
jQuery.Callbacks/fire@http://localhost/assets/vendor.js:3350:10
jQuery.Callbacks/self.fireWith@http://localhost/assets/vendor.js:3462:7
done@http://localhost/assets/vendor.js:9516:5
.send/callback@http://localhost/assets/vendor.js:9920:8

At which point I feel pretty stuck.

Any idea if there is something obvious I'm missing?

Am I doing something wrong in the .then() function? Or is this an Ember Data problem? Any idea where I start going in to debug that kind of error?