Getting Type Error when using Ember Embedded Records Mixin with keyForAttribute

I've been banging my head against deserializing data with Ember. I feel like I've set it up right but I keep getting the same error. I'm trying to use the EmbeddedRecords Mixin, but it simply hasn't worked for me. Below is my debug data.

DEBUG: Ember           : 1.6.1
DEBUG: Ember Data      : 1.0.0-beta.7+canary.b45e23ba
DEBUG: Handlebars      : 1.3.0
DEBUG: jQuery          : 1.10.2
DEBUG: Model Fragments : 0.2.2

Here is a simple setup of what I've been doing. I have my model defined like this -

App.Subject = DS.Model.extend({
  title: DS.attr('string'),
  sections: DS.hasMany('section')
});

App.Section = DS.Model.extend({
    title: DS.attr('string'),
    subject: DS.belongsTo('subject')
});

App.SubjectSerializer = DS.RESTSerializer.extend(DS.EmbeddedRecordsMixin, {
  attrs: {
    sections: { embedded: 'always' }
  }
});

and here is the format of the JSON payload I'm sending for a 'show'

{
    "subject": {
        "_id":"549987b098909eef0ac2d691",
        "title":"Maths",
        "sections":[{
            "title":"Precalc",
            "_id":"549987b098909eef0ac2d693"
        }, {
            "title":"Calc",
            "_id":"549987b098909eef0ac2d692"
        }],"__v":0
    }
}

I get the errors in the console

 Error while processing route: subjects.show undefined is not a function TypeError: undefined is not a function
at Ember.Mixin.create.extractSingle (http://localhost:3300/js/highered.js:2043:25)
at apply (http://localhost:3300/js/highered.js:20664:27)
at superWrapper [as extractSingle] (http://localhost:3300/js/highered.js:20240:15)
at Ember.Object.extend.extractFind (http://localhost:3300/js/highered.js:4007:21)
at Ember.Object.extend.extract (http://localhost:3300/js/highered.js:3892:37)
at http://localhost:3300/js/highered.js:11864:34
at invokeCallback (http://localhost:3300/js/highered.js:23228:19)
at publish (http://localhost:3300/js/highered.js:22898:9)
at publishFulfillment (http://localhost:3300/js/highered.js:23318:7)
at http://localhost:3300/js/highered.js:28736:9

highered.js:16581 undefined is not a function TypeError: undefined is not a function
at Ember.Mixin.create.extractSingle (http://localhost:3300/js/highered.js:2043:25)
at apply (http://localhost:3300/js/highered.js:20664:27)
at superWrapper [as extractSingle] (http://localhost:3300/js/highered.js:20240:15)
at Ember.Object.extend.extractFind (http://localhost:3300/js/highered.js:4007:21)
at Ember.Object.extend.extract (http://localhost:3300/js/highered.js:3892:37)
at http://localhost:3300/js/highered.js:11864:34
at invokeCallback (http://localhost:3300/js/highered.js:23228:19)
at publish (http://localhost:3300/js/highered.js:22898:9)
at publishFulfillment (http://localhost:3300/js/highered.js:23318:7)
at http://localhost:3300/js/highered.js:28736:9

Which as best I can tell is directly related to extractSingle at the this.keyForAttribute method

extractSingle: function(store, primaryType, payload, recordId, requestType) {
    var root = this.keyForAttribute(primaryType.typeKey),
        partial = payload[root];

    updatePayloadWithEmbedded(store, this, primaryType, partial, payload);

    return this._super(store, primaryType, payload, recordId, requestType);
},

although an interesting thing to note is that the error occurs at extractArray when I am using the subjects index route, which return the json above but with array brackets as well.

extractArray: function(store, type, payload) {
    var root = this.keyForAttribute(type.typeKey),
        partials = payload[pluralize(root)];

    forEach(partials, function(partial) {
      updatePayloadWithEmbedded(store, this, type, partial, payload);
    }, this);

    return this._super(store, type, payload);
}

Which makes me think that Ember Data is having trouble recognizing the format. This happens any time I define a serializer for a model, not just when I enable embedded records.

I'm hoping someone will be able to explain this. As a final note I've been using the Ember Data Model Fragments library as well, but I disabled that and still got this error so I don't think that is it.