There is a small bug in the calculation of max_length in CountryField when multiple=True:

kwargs["max_length"] = (
    len(self.countries)
    - 1
    + sum(len(code) for code in self.countries.countries)
)

I guess this is supposed to encode "Everything is selected" (the sum()) + "N-1 commas in between" (the len() - 1). However while self.countries.countries correctly lists all the possible countries, self.countries (i.e. Countries.__iter__!) emits the COUNTRIES_FIRST as well, so those are counted multiple times!

The fix will probably just be to replace len(self.countries) with len(self.countries.countries) here.