FINALIZE command for chunked video failing

[Manjit]

I am uploading video in chunks and then trying to call FINALIZE. Keep on getting:
“Large file can not be finalized synchronously.”

        result = new RequestBuilder(oauth, "POST", "https://upload.twitter.com/1.1/media/upload.json")
            .AddParameter("command", "FINALIZE")
            .AddParameter("media_id", initResult.media_id_string)
            .Execute2().Result;

//code from Execute2
            try
            {
                response = await request.GetResponseAsync().ConfigureAwait(false);
                using (var stream = response.GetResponseStream())
                {
                    using (var reader = new StreamReader(stream))
                    {
                        content = reader.ReadToEnd();
                    }
                }
            }
            catch (WebException ex)
            {
                using (var stream = ex.Response.GetResponseStream())
                {
                    using (var reader = new StreamReader(stream))
                    {
                        content = reader.ReadToEnd();
                    }
                }
                throw;
            }
            finally
            {
                if (response != null)
                {
                    ((IDisposable)response).Dispose();
                }
            }