Asp.Net MVC encrypted querystring

I am trying to get an encrypted querystring on a ActionResult but the parameter is always null.

I have implemented an e-mail URL and encrypted some informations like this:

string key = "r0b1nr0y";
var queryString = EncryptDecryptQueryString.Encrypt(String.Format("testId={0}&otherInfo={1}", Id, otherInfo), key);

It is resulting in this URL parameter:

?fpZG2mFDOZbuuBFccKLeu9Rzbn/I05i577IaaMSt0uztuHmWdeVIOQ==

And the url is something like this:

www.test.com/MyController/MyAction?fpZG2mFDOZbuuBFccKLeu9Rzbn/I05i577IaaMSt0uztuHmWdeVIOQ==

Now, at the controller side, the ActionResult is suppose to receive a string.

Running the program, when i try to test the generated url, the code reach MyAction but the string parameter is always null.

public ActionResult MyAction(string queryString)
{
     ... Do Stuff here...        
}

I have tried to create a specific route for it as well(like this example), but didn't work.

routes.MapRoute(
            "RouteTest",
            "MyController/MyAction/{queryString}",
            new { controller = "MyController", action = "MyAction", queryString = "" }
        );

I can take the whole url with Request.RawUrl, but i really can't understand why the encrypted string parameter is null(or empty).

Have i missed the point somewhere or is there an explanation for the null parameter value?