[MEncoder-users] duration lost after flv transcoding

[oisurge38-web at yahoo.fr]

Hi,
I got an issue while trying to transcode an avi file to flv.
I use this command :
mencoder -of lavf -ovc lavc -lavcopts vcodec=flv -lavfopts
i_certify_that_my_video_stream_does_not_use_b_frames -o output.flv
input.avi

The resulting flv file doesn't have duration information (I got it with ffmpeg).
I thought that setting the ofps (matching the input fps) could help, but it did not.

How could I resolve that ?

Thanks