URL rewriting to be done for aspx page without extension?

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C#

C#4.0

I tried to remove the .aspx extension in the url bar for my web page but it is showing error my code is,

global.asax file:

void Application_Start(object sender, EventArgs e)
       {
           RegisterRoutes(RouteTable.Routes);
       }

       private void RegisterRoutes(RouteCollection routeCollection)
       {
           routeCollection.MapPageRoute("RouteForCustomer", "Customer/{Id}", "~/Customer.aspx");

       }

my web page code is:

protected void Page_Load(object sender, EventArgs e)
        {
            try
            {
                    string id = Page.RouteData.Values["Id"].ToString();
                    Response.Write("<h1>Customer Details page</h1>");
                    Response.Write(string.Format("Displaying information for customer : {0}", id));
                }
            }
            catch (Exception ex)
            {
                throw ex;
            }
        }

error is:

object reference not set to an instance of an object. In the line string id in customer.aspx.