ASP.NET – ActionResult parameter is coming back null always when passing string – why?

I am barely starting out with my first project on the ASP.NET MVC project type and I am creating a Details page where instead of passing the templated (int id), I would like to pass a string instead. But when I am in debug mode, and enter this in the URL, “myString” is null. Why so? Do I have to change anything else somehwere else?