Why are these two Actions considered ambiguous despite the parameters being different?

The current request for action 'Index' on controller type 'UsersController' is ambiguous between the following action methods: System.Web.Mvc.ActionResult PostUser(Models.SimpleUser) on type Controllers.UsersController System.Web.Mvc.ActionResult PostUser(Int32, Models.SimpleUser) on type Controllers.UsersController

Is happening when I try to POST website.com/users/ with form values.

When there is NO ID (website.com/users/) I want it to create a new user, when there IS an ID (/users/51 for example) I want it to update that user, so how can I make it tell the difference between these two Actions?

Here are the two actions:

    [HttpPost]
    [ActionName("Index")]
    public ActionResult PostUser(SimpleUser u)
    {

        return Content("User to be created...");
    }

    [HttpPost]
    [ActionName("Index")]
    public ActionResult PostUser(int id, SimpleUser u)
    {

        return Content("User to be updated...");
    }

Here is the MapRoute:

    routes.MapRoute(
        "Users",
        "users/{id}",
        new { controller = "Users", action = "Index" }
    );