Facter.add(:foo, :setcode => nil) on facter 3 fails with:
2020-05-28 09:53:17.828663 ERROR puppetlabs.facter - error while resolving custom facts in /root/custom_facts/fact.rb: unexpected option setcode
On facter 4 fails with:
unable to add resolve nil for fact oss: Invalid resolution options [:setcode]