C#关键字as出现的错误

 　　　　　　ObjectCache cache = MemoryCache.Default;

            //
            string cacheData1 = cache["key1"] as string;//得不到数据
            //2
            //string cacheData1 = Convert.ToString(cache["key1"]);//这个ok

            CacheItemPolicy policy1 = new CacheItemPolicy();
            policy1.AbsoluteExpiration = new DateTimeOffset(DateTime.Now.AddMinutes());
            cache.Set("key1", , policy1);

            Response.Write($"<h1>{cacheData1}</h1>");
            Response.End();