granulepos of some frames is -1.

I made the ogv file by using ffmpeg2theora version 0.23 with Windows XP.

I succeeded in the decoding.
However, granulepos is strange.
granulepos of some frames is -1.
The frame number becomes strange because of that, too.
When granulepos of the first key frame is -1, the frame number is begun
from -1.

I am resetting the frame number by using TH_DECCTL_SET_GRANPOS of each frame. In that case, the frame number shifts when the frame of correct granulepos appears.

Am I misunderstanding anything?