Asp.net mvc 2 - Error redirecting from ActionFilter?

Question

What changed from mvc1 and mvc2? I have the following code that redirects to a login page if the user has not been authenticated. This doesn't work with mvc2 and results in "System.Web.HttpException: Cannot redirect after HTTP headers have been sent"

public class RequiresAuthenticationAttribute : FilterAttribute, IAuthorizationFilter
{
  public void OnAuthorization(AuthorizationContext filterContext)
  {
    if (!filterContext.HttpContext.User.Identity.IsAuthenticated)
    {
      string redirectOnSuccess = filterContext.HttpContext.Request.Url.AbsolutePath;
      string redirectUrl = string.Format("?ReturnUrl={0}", redirectOnSuccess);
      string loginUrl = System.Web.Security.FormsAuthentication.LoginUrl + redirectUrl;
      filterContext.HttpContext.Response.Redirect(loginUrl, true);
    }
  }
}

A stack track is as follows:

System.Web.HttpException: Cannot redirect after HTTP headers have been sent.
  at System.Web.HttpResponse.Redirect(String url, Boolean endResponse)
  at System.Web.HttpResponseWrapper.Redirect(String url, Boolean endResponse)
  at System.Web.Mvc.RedirectResult.ExecuteResult(ControllerContext context)
  at System.Web.Mvc.ControllerActionInvoker.InvokeActionResult(ControllerContext controllerContext, ActionResult actionResult)
  at System.Web.Mvc.ControllerActionInvoker.<>c__DisplayClass14.<InvokeActionResultWithFilters>b__11()
  at System.Web.Mvc.ControllerActionInvoker.InvokeActionResultFilter(IResultFilter filter, ResultExecutingContext preContext, Func`1 continuation)
  at System.Web.Mvc.ControllerActionInvoker.<>c__DisplayClass14.<>c__DisplayClass16.<InvokeActionResultWithFilters>b__13()
  at System.Web.Mvc.ControllerActionInvoker.InvokeActionResultWithFilters(ControllerContext controllerContext, IList`1 filters, ActionResult actionResult)
  at System.Web.Mvc.ControllerActionInvoker.InvokeAction(ControllerContext controllerContext, String actionName)