ffmpeg codec input stream subtitles

When trying to convert an .mkv file with .ass subtitles, ffmpeg gave me the following error:

decoder: codec not found for input stream

turns out, it fails to detect and convert the subtitles coded. there are two options that i know of:

• disable the subtitles for that video file, use the “-sn” option
• simply copy the subtitles codec, use “-scodec copy”

source & more info: https://en.wikibooks.org/wiki/FFMPEG_An_Intermediate_Guide/subtitle_options#Set_Subtitle_Codec