I am trying to learn how to use IContentFinder for a page not found scenario.

Umbraco v6.2.1 (Assembly version: 1.0.5261.28997)

Trying to hook it into the application start up, I get an error:

Server Error in '/' Application.

---

Resolution is frozen, it is not possible to configure it anymore.

Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code. 

Exception Details: System.InvalidOperationException: Resolution is frozen, it is not possible to configure it anymore.

Source Error: 

Line 9:           //On application starting event...
Line 10:            //Add to the ContentFinder resolver collection our custom 404 Content Finder resolver
Line 11:          ContentLastChanceFinderResolver.Current.SetFinder(new NotFoundContentFinder());
Line 12:             base.ApplicationStarting(umbracoApplication, applicationContext);
Line 13:         }

here is my code:

using Umbraco.Core;
using Umbraco.Web.Routing;

namespace WHRX.Helpers
{
    public class RegisterEvents : ApplicationEventHandler {
        protected override void ApplicationStarted(UmbracoApplicationBase umbracoApplication, ApplicationContext applicationContext) {
           //On application starting event...
         //Add to the ContentFinder resolver collection our custom 404 Content Finder resolver
          ContentLastChanceFinderResolver.Current.SetFinder(new NotFoundContentFinder());
            base.ApplicationStarting(umbracoApplication, applicationContext);
        }
    }
}

Feeling stumped and having a really hard time implementing this!