求救:缓存写入失败

Question

            
            string cacheName = "ask_model_" + Id.ToString();
            model = HttpContext.Current.Cache[cacheName] as Ask;
            if (model == null)
            {
                model = bll.GetModel(Id);
                if (model == null)
                {
                    Response.Redirect("error.aspx");
                    Response.End();
                }                Response.Write("丫的Model空啊<br/>");
                HttpContext.Current.Cache.Add(cacheName, model, null,Cache.NoAbsoluteExpiration, new TimeSpan(0, 0, 30), CacheItemPriority.Default, null);//缓存30秒            }
          将model写入缓存为何会失败呢？每次都会执行 Response.Write("丫的Model空啊<br/>");  同样的方法在另外一个页面又没问题,郁闷了....