WebException在消息错误时返回null

Question

我正在尝试从400错误请求中捕获错误消息。在邮递员中，我收到400个错误的请求，而从端点获得的正文是：

{
    "error": {
        "message": "400 - Client no valid",
        "type": "USER_ERROR"
    },
    "location": "..."
}

所以我试图在代码中捕获该错误，如下所示：

catch (WebException ex)
{
   using (WebResponse response = ex.Response)
   {
       HttpWebResponse httpResponse = (HttpWebResponse)response;
       Console.WriteLine("Error code: {0}", httpResponse.StatusCode);
       using (Stream data = response.GetResponseStream())
       using (var reader = new StreamReader(data))
       {
          string text = reader.ReadToEnd();
          OnError(AppResources.ErrorTitle, text);
       }
    }
}

但是“文本”始终为“”，如何从400错误的请求中获取错误正文？