Seems like misconfigured routing. Your RegisterRoutes method in Global.asax.cs should look like this:
public static void RegisterRoutes(RouteCollection routes)
{
routes.IgnoreRoute("{resource}.axd/{*pathInfo}");
routes.MapRoute(
"Default", // Route name
"{controller}/{action}/{id}", // URL with parameters
new { controller = "Account", action = "Login", id = "" } // Parameter defaults
);
}
In line "{controller}/{action}/{id}" presence of {id} means that is going to be substituted by it's value.
Any other parameter that is not present in routing string would be decoded as ?some_param=value