.net core下上传文件报错

internal static void AddInternal(WebHeaderCollection headers, string key, string value)
        {
           ......
                var mi = typeof(WebHeaderCollection).GetMethod(
                    internalMethodName,
                    BindingFlags.NonPublic | BindingFlags.Instance,
                    null,
                    new Type[] { typeof(string), typeof(string) },
                    null);
                ////// mi此时为空，所有后续操作是提示对象为空
                _addInternalMethod = mi;
            }
           _addInternalMethod.Invoke(headers, new object[] { key, value });
        }



哪位知道如何解决啊， 网上没查到资料