Upgrade resulted in an error for every Ref parameter

I have been running a an MVC 5 site and wanted to upgrade to 5.2.3. When I install the 5.2.3 nuget package and try to run the site that was working I now get an error in the RouteCOnfig file, "routes.MapMvcAttributeRoutes();"

There is an error for every method call that has a ref parameter the error is always something like:

Cannot call action method '{what ever method has ref param}' on controller 'AllThings.WebUI.Controllers.PostController' because the parameter 'AllThings.Entities.Post& post' is passed by reference. Parameter name: methodInfo

What am I missing?

Code sample

public class RouteConfig
{
    public static void RegisterRoutes(RouteCollection routes)
    {
        routes.IgnoreRoute("{resource}.axd/{*pathInfo}");

        routes.MapMvcAttributeRoutes();

        routes.MapRoute(
            name: "Default",
            url: "{controller}/{action}/{id}",
            defaults: new { controller = "Home", action = "Index", id = UrlParameter.Optional }
        );

        routes.MapRoute(null, "{controller}/{action}");


    }
}

I have cut out most of the code so the ref method call can be easily seen on the setpostcoredetails call.

[Route("~/{site}/edit")]
    [HttpPost]
    [Authorize]
    public ActionResult Edit(string site, PostViewModel postViewModel)
    {
            IPostRepository postRepository = contextFactory.GetPostRepository();
            Post post = postRepository.Posts.FirstOrDefault(i => i.PostId == postViewModel.PostId);

            SetPostCoreDetails(ref post, User.Identity.GetUserId(), postViewModel);

            LoadEditUIOptions(postViewModel);
            return View(postViewModel);
        }
    }



private void SetPostCoreDetails(ref Post post, string userId, PostViewModel postViewModel)
    {

    }