How do I display a custom error page for an Http 401 result?

I have a controller with an Authorize attribute like this:

[Authorize(Roles = "Viewer")]
public class HomeController : Controller
{
   //...
}

and my web.config has customErrors set like the following:

<customErrors mode="On">
      <error statusCode="401" redirect="notauthorized.html"/>
  </customErrors>

When I try to invoke an action on the Home controller using a non-authorized role I just get a blank page. I don't get redirected to the custom page. Any ideas?