Can not embed a watermak and a subtitle at the same time

[LiveIsLive]

This is my code , as the result of running the code, the output file only contains the watermark,but without subtitle.

Xabe.FFmpeg.IMediaInfo inputFile = await Xabe.FFmpeg.FFmpeg.GetMediaInfo(@"Test.avi");
Xabe.FFmpeg.IVideoStream video = inputFile.VideoStreams.First().AddSubtitles(@"Test.srt");
video = video.SetWatermark(@"Test.png", Xabe.FFmpeg.Position.BottomRight);
Xabe.FFmpeg.IConversion conversion = Xabe.FFmpeg.FFmpeg.Conversions.New().AddStream(video).SetOutput(@"Test.mp4").SetOverwriteOutput(true);
await conversion.Start();