Unit test for HtmlHelper throwing Object reference not set to an instance of an object

I have created a "Mock" HtmlHelper to hold my model in the ViewDataDictionary. When my test executes the HtmlHelper.TestBoxFor() throws a null reference exception. What am I missing here? ... I thought I had everything mocked up properly.

HtmlHelper Extension

    public static MvcHtmlString TextBoxForNonZeroUInt<TModel, TProperty>(this HtmlHelper<TModel> helper,
                                                                         Expression<Func<TModel, TProperty>> expression)
    {
        var value = ModelMetadata.FromLambdaExpression(expression, helper.ViewData).Model;
        if (expression.Body.Type == typeof(uint) && (uint)value == 0)
        {
            return helper.TextBoxFor(expression, new { Value = "" });
        }
        return helper.TextBoxFor(expression);
    }

Test

[TestClass]
public class TextBoxForNonZeroUIntTests
{
    private HtmlHelper<T> CreateHtmlHelper<T>(ViewDataDictionary viewData)
    {
        var cc = new Mock<ControllerContext>(
            new Mock<HttpContextBase>().Object,
            new RouteData(),
            new Mock<ControllerBase>().Object);


        var mockViewContext = new Mock<ViewContext>(
            cc.Object,
            new Mock<IView>().Object,
            viewData,
            new TempDataDictionary(),
            new Mock<StringWriter>().Object);

        var mockViewDataContainer = new Mock<IViewDataContainer>();
        mockViewDataContainer.Setup(view => view.ViewData).Returns(viewData);

        return new HtmlHelper<T>(mockViewContext.Object, mockViewDataContainer.Object);
    }

    private class TestSiteDisplayModel
    {
        public uint SiteId { get; set; }
        public string Name { get; set; }
    }

    [TestMethod]
    public void NonUIntWithoutValue_ShouldReturnDefaultTextBoxFor()
    {
        var viewData = new ViewDataDictionary<TestSiteDisplayModel>
        {
            Model = new TestSiteDisplayModel {SiteId = 0, Name = "POS BETTER WORK"}
        };
        var htmlHelper = CreateHtmlHelper<TestSiteDisplayModel>(viewData);

        var result = htmlHelper.TextBoxForNonZeroUInt(model => model.SiteId);
    }
}

}