Filtering is not allowed

Your code seems to be working for me when used in a brand new ASP.NET MVC 5 application created in VS2013 (the regular expressions may need to be adjusted, but that's a minor detail). I've uploaded a full solution here. Could you try it out?

To be clear, I noticed that you've tagged the question with asp.net-mvc-3 and asp.net-mvc-4, but I haven't had the opportunity to test on those version of ASP.NET MVC.

Try to add null-check before apply filter:

public override void OnActionExecuting(ActionExecutingContext filterContext)
{
    var response = filterContext.HttpContext.Response;

    if (response.Filter == null) return; // <-----

    response.Filter = new YourFilter(response.Filter);
}