Testing backbone model extend from another model with mocha

When I run my tests (I am using Mocha, Chai and Sinon) for a backbone model which extends from another backbone model, I get an error:

TypeError: 'undefined' is not an object (evaluating 'Model._entity')

I thought that by instantiating Model._entity that this error would go away, but it doesn't. Should I be using a stub instead?

I've searched the web for an answer but all the examples are for models that extend directly from Backbone and not another model.

What do I need to do to avoid this error?

My test code:

describe("Model.address", function () {

    beforeEach(function () {

        // build up a fake server
        this.server = sinon.fakeServer.create();

        this.server.autoRespond = true;

        this.parentModel = new Model._entity();

        this.Model_address = new Model.Address();
    });


    afterEach(function () {

        // delete fake server
        this.server.restore();
    });

    describe("Defaults", function () {

        // test defaults
        it("has default values", function () {
            // create empty model
            var model = this.Model_address;

            // expect model to be truthy
            expect(model).to.be.ok;


        });

    });
});