Forced subtitles loose their forced flag when converting with avconv, even when just copying streams:

avconv -i input.mkv \
-map 0:0 -map 0:1 -map 0:2 -map 0:3 -map 0:4 \
-c:v:0 copy \
-c:a:0 copy \
-c:s:0 copy \
-c:s:1 copy \
output.mkv
Is this a bug?
Is there an option to set the forced flag for subtitle streams?