ffmpeg -i no-subtitles.mkv -map 0:s:0 -c:s copy empty.srt
exit=4294967274
Input #0, matroska,webm, from 'no-subtitles.mkv':
  Metadata:
    COMPATIBLE_BRANDS: isomiso2avc1mp41
    MAJOR_BRAND     : isom
    MINOR_VERSION   : 512
    ENCODER         : Lavf61.7.100
  Duration: 00:00:04.00, start: 0.000000, bitrate: 34 kb/s
  Stream #0:0: Video: h264 (High), yuv420p(progressive), 960x540, 24 fps, 24 tbr, 1k tbn (default)
      Metadata:
        HANDLER_NAME    : VideoHandler
        VENDOR_ID       : [0][0][0][0]
        ENCODER         : Lavc61.19.100 libx264
        DURATION        : 00:00:04.000000000
Stream map '' matches no streams.
To ignore this, add a trailing '?' to the map.
Failed to set value '0:s:0' for option 'map': Invalid argument
Error parsing options for output file empty.srt.
Error opening output files: Invalid argument