ASP.NET MVC - passing parameters to the controller

I have a controller with an action method as follows:

public class InventoryController : Controller
{
    public ActionResult ViewStockNext(int firstItem)
    {
        // Do some stuff
    }
}

And when I run it I get an error stating:

The parameters dictionary does not contain a valid value of type 'System.Int32' for parameter 'firstItem'. To make a parameter optional its type should either be a reference type or a Nullable type.

Your routing needs to be set up along the lines of {controller}/{action}/{firstItem}. If you left the routing as the default {controller}/{action}/{id} in your global.asax.cs file, then you will need to pass in id.

routes.MapRoute(
    "Inventory",
    "Inventory/{action}/{firstItem}",
    new { controller = "Inventory", action = "ListAll", firstItem = "" }
);

... or something close to that.