[Libav-user] Problem encoding audio to ogg/vorbis

[hipersayan x]

information of the Environment:

Operating system: Arch Linux
ffmpeg version: 20110330

I'm trying to encode audio to ogg/vorbis format, in the
output-example.c file, if I set the output name to video.ogv, the
audio is encoded in flac by default.
Then I tried putting the line:

fmt->audio_codec = CODEC_ID_VORBIS;

After the line:

fmt = av_guess_format(NULL, filename, NULL);

When I compile and run the example, it return the error:

output-example: libavutil/mathematics.c:79: av_rescale_rnd: Assertion
`c > 0' failed.

Also, if I set the output name to video.webm, it uses libvorbis by
default, returns the same error.
My question is, do I need to set another parameter to encode the audio
stream as vorbis?
Thanks in advance.