I have this set of routes:

        routes.MapRoute(
            "IssueType",
            "issue/{type}",
            new { controller = "Issue", action = "Index" }
        );

        routes.MapRoute(
            "Default", // Route name
            "{controller}/{action}/{id}", // URL with parameters
            new { controller = "Home", action = "Index", id = UrlParameter.Optional } // Parameter defaults
        );

Here is the controller class:

public class IssueController : Controller
{
    public ActionResult Index()
    {
        // todo: redirect to concrete type
        return View();
    }

    public ActionResult Index(string type)
    {
        return View();
    }
}

why, when i request http://host/issue i get The current request for action 'Index' on controller type 'IssueController' is ambiguous between the following action methods:
I expect that first one method should act when there is no parameters, and second one when some parameter specified.

where did i made mistake?

UPD: possible duplicate: http://stackoverflow.com/questions/436866/can-you-overload-controller-methods-in-asp-net-mvc