[Ffmpeg-user] Negative size returned by avcodec_decode_audio

I'm decoding a fairly standard mpeg-2 stream with avcodec CVS from a couple of weeks ago. This call sometimes returns a negative value for decode_size. e.g. len would be 1688 and decode_size would be -128. The comment for avcodec_decode_audio seems to imply that a negative value here is wrong (i.e. it should be the size of the data or 0, not sure why it's a signed value...) len = avcodec_decode_audio(codec_context, buf, &decode_size, packet->data, packet->size); Can anyone shed any light on this? Thanks,