远程服务器返回错误：（400）错误的请求。 apiRequest.ChallengedUri'引发了类型为'System.NullReferenceException'的异常

Question

我正在尝试从网址中获取响应，该网址必须是JSON，如下所示：

我使用此代码

if (cities != null)
            {
                /*Calling API http://openweathermap.org/api */
                string apiKey = "xxxxxx";
                HttpWebRequest apiRequest =
                WebRequest.Create("http://api.openweathermap.org/data/2.5/weather?id=" +
                cities + "&appid=" + apiKey + "&units=metric") as HttpWebRequest;



                string apiResponse = "";
                using (HttpWebResponse response = (HttpWebResponse)apiRequest.GetResponse() as HttpWebResponse)
                {
                    StreamReader reader = new StreamReader(response.GetResponseStream());
                    apiResponse = reader.ReadToEnd();
                }
                /*End*/

所以我在HttpWebResponse response =（HttpWebResponse）apiRequest.GetResponse（）作为HttpWebResponse时遇到错误

ChallengedUri ='apiRequest.ChallengedUri'引发了类型为'System.NullReferenceException'的异常