Looks like this commit, in particular the changes to spell/dictionary.py.
I was able to make it fail by creating a user dictionary. Replacing line 40 with what it was fixes it.
Code:
self.words = {(w, langcode) for w, langcode in kwargs['words']}
I wonder if any of the other changes in that commit will have the same problem.
EDIT: I'm not making a PR. There is some weirdness with character encoding for the file and I don't want to break anything. I converted it to UTF8 to test it, which changed lines 464 and 469.