C＃WebClient挂起而不是抛出错误

Question

我正在尝试将JSON发布到Qualtrics API以发送调查请求。

当所有必要的ID都有效并且我收到如下响应时，这可以正常工作。

{"result":{"id":"EMD_0xpOmBl99999999"},"meta":{"httpStatus":"200 - OK","requestId":"a075f677-bbcf-45fb-bb7d-328999999999"}}

如果我将其中一个值更改为无效ID，则API应返回“400 Bad request”错误，并显示一条消息，告诉您哪个值不正确，例如

{"meta":{"httpStatus":"400 - Bad Request","error":{"errorMessage":"Invalid surveyId parameter. surveyLink must contain a valid survey","errorCode":"JP_2"},"requestId":null}}

但是，在我的C＃应用​​程序中，我没有收到预期的400错误，代码只是无限期挂起在WebClient.UploadString步骤上。

如果使用Poster等测试工具发布相同的JSON，我会得到预期的响应。

我该如何解决这个问题？

private static string TestSendQualtricsSurvey()
{
    string apiUrl = @"https://eu.qualtrics.com/API/v3/distributions";
    string listId = "ML_123123123";
    string surveyId = "SV_123123123";
    string libraryId = "UR_123123123";
    string messageId = "MS_123123123";
    string senderName = "Survey Sender";
    string senderEmail = "testing@email.com";
    string emailSubject = "Please take this survey";
    string contactId = "MLRP_123123123";
    string apiKey = "123123123";

    Survey survey = new Survey();
    survey.surveyLink = new SurveyLink() { surveyId = surveyId, type = "Individual" };
    survey.header = new Header() { fromEmail = senderEmail, fromName = senderName, subject = emailSubject };
    survey.message = new Message() { libraryId = libraryId, messageId = messageId };
    survey.recipients = new Recipients() { contactId = contactId, mailingListId = listId };
    survey.sendDate = DateTime.UtcNow.ToString("yyyy-MM-ddTHH:mm:ssZ");

    using (var ms = new MemoryStream())
    {
        var js = new DataContractJsonSerializer(typeof(Survey));
        js.WriteObject(ms, survey);
        ms.Position = 0;
        var sr = new StreamReader(ms);                
        string surveyJson = sr.ReadToEnd();

        using (var client = new WebClient())
        {
            client.Headers[HttpRequestHeader.ContentType] = "application/json";
            client.Headers.Add("X-API-TOKEN", apiKey);

            string response;

            try
            {
                //IT HANGS HERE
                response = client.UploadString(apiUrl, "POST", surveyJson);
                return response;
            }
            catch (WebException ex)
            {
                using (var reader = new StreamReader(ex.Response.GetResponseStream()))
                {
                    response = reader.ReadToEnd();
                }
                return response;
            }
        }
    }            
}