Thanks for the report. That code seems to be quite old in style in general; it appears to use obarrays as some kind of poor man's hash table, so perhaps it even pre-dated the addition of hash tables to Emacs.

In any case, filling a vector with `nil` never was a correct was to make an obarray. It must be filled with the integer 0. Of course, today `obarray-make` is preferable.

Do you know if the Github repository (hrs-allbsd/tamago) is the closest we can get to an upstream source for this package? If so, open a bug report (issue) there. We will help explaining the problems.

Attached is an (untested) patch against Tamago. Maybe that will help you get by.