Referencing Parameterized Index method in ASP.NET MVC Returns Null

I am writing tests using SpecFlow and ASP.NET MVC and the code steps are perfectly fine. However, I wanted to write tests based on the validation on the model.

In my HomeController (for a login screen), the Index() method is implemented twice, both with different method signatures:

// When the page is initially loaded, the value of null is passed to 
// the parameter. This would prompt the view to load the login-screen. 
public ActionResult Index(string u) {
    // some declaration
    return View(model);
}

[HttpPost, ValidateInput(false)]
public ActionResult Index(myobject o) {
    // some declaration
    return View(model);
}

// No default implementation for Index() 

public string SomeSampleFunction() {
    return "This is a test string";
}

When I write my tests, every reference to the Index() method retuns null. I tried doing such as:

var _controller = new HomeController();
var _result = _controller.Index("") as ViewResult;
// The succeeding line also returns null
// var _result = _controller.Index(string.empty) as ViewResult;
Assert.IsInstanceOf<ViewResult>(_result);
Assert.IsEmpty(((ViewResult)_result).ViewName);
Assert.AreEqual("Log In", homeController.ViewData["Title"], "Page title is wrong");

Am I missing something on this one? The references to the Index() method is always null but if I call an ordinary method such as "SomeSampleFunction", it works just fine. Any help?