IdependencyResolver is always null in unit test using moq

I've implemented ninject in a mvc project and this works as expected now I'm wanting to unit test, but can't seem to get it to work:

controller:

 private IDependencyResolver _resolver;

 public HomeController(IDependencyResolver resolver)
 {
        _resolver = resolver;
 }

  public ActionResult Index()
  {
        var model = _resolver.GetService<ISignUpViewModel>();
        return PartialView("Login", model);
  }

Unit Test

    private IDependencyResolver _resolverMock;

    [TestMethod]
    public void SignUpTest()
    {

        var ctrl = new HomeController(_resolverMock);
        var signUpMock = new Mock<ISignUpViewModel>();

        ctrl.Index();
        ctrl.ViewData.Model = signUpMock;

        //Assert.AreEqual("", "");

    }

The resolver is always null.

Ive looked at Ninject.MockingKernel.Moq should I set the resolver in there?

Any idea where I'm going wrong?

Thanks