using a mocked controller context to test a controller

I am trying to run unit tests against a controller decorated with the [Authorize] attribute. The solutions I have found on SO indicate that I should use Moq to mock the authorization. The solution code can be found here.

var controller = new UserController();
var mock = new Mock<ControllerContext>();
mock.SetupGet(x => x.HttpContext.User.Identity.Name).Returns("SOMEUSER");
mock.SetupGet(x => x.HttpContext.Request.IsAuthenticated).Returns(true);
controller.ControllerContext = mock.Object;

I have implemented the above solution in my test:

var controller = new HomeController();
var mock = new Mock<ControllerContext>();
mock.SetupGet(x => x.HttpContext.User.Identity.Name).Returns("SOMEUSER");
mock.SetupGet(x => x.HttpContext.Request.IsAuthenticated).Returns(true);
controller.ControllerContext = mock.Object;

var result = controller.GetStarted();
result.ExecuteResult(mock.Object);
Assert.IsTrue(false); //I am running with a breakpoint here

Result generates, but when I try to execute the result with the mocked controllerContext, I get an error. How do I use the mocked ControllerContext to test the controller?

The skeleton code for the controller is:

[Authorize]
public ActionResult GetStarted()
{
    if (User.Identity.IsAuthenticated)
    {
        var user = CommonUtil.Instance.UserManager.FindByName(User.Identity.Name);
        if (user != null)
        {
            ViewBag.IsAdministrator = user.Roles.Contains("Administrators");
            ViewBag.IsActiveUser = user.Roles.Contains("ActiveUsers");
        }
    }
    return View();
}

I have a feeling this is a trivial fix, but I have no idea what I am doing, so it seems impossible.