IndexError: list index out of range?

[conanchen]

% webvtt segment introduction-1.webvtt --output t1
Traceback (most recent call last):
File "/usr/local/bin/webvtt", line 8, in
sys.exit(main())
File "/usr/local/lib/python3.9/site-packages/webvtt/cli.py", line 27, in main
segment(
File "/usr/local/lib/python3.9/site-packages/webvtt/cli.py", line 47, in segment
WebVTTSegmenter().segment(f, output, target_duration, mpegts)
File "/usr/local/lib/python3.9/site-packages/webvtt/segmenter.py", line 93, in segment
self._slice_segments(captions)
File "/usr/local/lib/python3.9/site-packages/webvtt/segmenter.py", line 45, in _slice_segments
self.segments[i].append(c)
IndexError: list index out of range