implicit conversion from nil

[yspreen]

This is happening for part two in 2016, day 18.
I could fix the issue (day.md was not updating) by running -r:

 aoc -s 19991126
Correct! This day is now complete!
- Updating calendar...
no implicit conversion from nil to integer
(.pyenv) ➜  18 git:(main) ✗ aoc -r
- Updating puzzle...