Getting an Object not set exception as I uncomment Export

[Export(typeof(IAuthenticationService))]

  [Export(typeof(IUserService))]

  [PartCreationPolicy(CreationPolicy.Shared)]

  public class myAuthenticationService : AuthenticationService<myEntities>, IUserService

  {

         public IFUser CurrentUser

      {

          get { return Principal as IFUser; }

      }

      protected override void OnLoggedIn()

      {

          base.OnLoggedIn();

          EventFns.Publish(new LoggedInMessage(CurrentUser));

      }

      protected override void OnLoggedOut()

      {

          base.OnLoggedOut();

          EventFns.Publish(new LoggedOutMessage());

      }

  }