How to test ASP.NET MVC routines with optional parameters

I defined a map route in Global.asax.cs as below

routes.MapRoute(null,
    "product/{category}",
    new { controller = "Product", action = "List", category = UrlParameter.Optional });

and ProductController's List method

public ActionResult List(string category = null)
{
    IList<Product> products;

    if (category == null)
    {
        products = _productRepository.GetAll();
    }
    else
    {
        products = _productRepository.GetAllByCategory(category);
    }

    return View(products);
}

And here are a few attempts to test the route (with MVC Contrib Test Helper support)

First attempt

    "~/product".Route().ShouldMapTo<ProductController>(c => c.List());

I have a compile error: "An expression tree may not contain a call or invocation that uses optional arguments". Grrrr, expressions don't play with optional arguments.

Second attempt

    "~/product".Route().ShouldMapTo<ProductController>(c => c.List(null));

The test throws a MvcContrib.TestHelper.AssertionException: "Value for parameter 'category' did not match: expected 'System.Web.Mvc.UrlParameter' but was ''."

Third attempt

    "~/product".Route().ShouldMapTo<ProductController>(c => c.List(""));

Same result as second attempt. Grrrr...

And the forth attempt

    var routeData = "~/product".WithMethod(HttpVerbs.Get);
    routeData.Values["category"] = null;
    routeData.ShouldMapTo<ProductController>(c => c.List(null));

Now the test pass.

Why? 'category' is an optional parameter, as mentioned here, I expect that I don't need to explicit to define it in route value dictionary and the third attempt should be passed. Is a place to improve MVC Contrib test helper?