OWA Custom Authentication Module

Developing a custom authentication module for Outlook Web Access (OWA) 2010. For now, this module is always going to log in with a static userid (jdoe@staging.local); i.e. anybody accessing the OWA site should be automatically logged in with the jdoe id. Once I get this working, I can make the necessary mods.

I have developed a C Sharp class library that I think performs this. Code given below:

namespace MyAuth
{
    public class AuthModule : IHttpModule
    {
        public void Dispose()
        {
            throw new NotImplementedException();
        }

        public void Init(HttpApplication context)
        {
            context.PreRequestHandlerExecute += new EventHandler(this.OnAuthenticateRequest);
        }

        public void OnAuthenticateRequest(Object source, EventArgs e)
        {
            HttpApplication app = (HttpApplication)source;

            if (app.Context.User.Identity.IsAuthenticated == false)
            {
                WindowsIdentity identity = new WindowsIdentity("jdoe@staging.local");
                WindowsPrincipal principal = new WindowsPrincipal(identity);
                app.Context.User = principal;
            }
        }
    }
}

I have installed the module successfully. My problem is that the app crashes (server error) at the line

app.Context.User = principal.

If I comment that line (which basically means the module does nothing), the module works just fine. Any ideas on what I might be doing wrong ?