.NET使用OSS上传文件时出现异常

经调试，在读取上传回调返回的消息内容时发现putObjectResult.ResponseStream为null 代码片段：

// 读取上传回调返回的消息内容
private static string GetCallbackResponse(PutObjectResult putObjectResult)
   {
            string callbackResponse = null;
            using (var stream = putObjectResult.ResponseStream)
            {
                var buffer = new byte[4 * 1024];
                var bytesRead = stream.Read(buffer, 0, buffer.Length);
                callbackResponse = Encoding.Default.GetString(buffer, 0, bytesRead);
            }
            return callbackResponse;
        }

//断点续传上传文件
UploadObjectRequest request = new UploadObjectRequest(bucketName, objectName, localFilename)
    {
           PartSize = 8 * 1024 * 1024,
           ParallelThreadCount = 3,
           CheckpointDir = checkpointDir,
     };
var result = client.ResumableUploadObject(request);
responseContent = GetCallbackResponse(result);

调试截图：

本人水平有限，只知道这个ResponseStream为null引发了这个异常，但具体也不知道为什么会为null，以及该怎么处理，希望各位路过的大佬指导一下，谢谢