Not your fault, something has been odd since a recent rework of culture mechanic, city plots can now for some reason have ZERO culture which kinda means it is neutral land with a city on it. The error should be relatively rare, and its only consequence is that the limited python script the error happens in stops prematurely so that assimilation of rebels won't happen that turn at least.
I think the preconditions for the error should be quite rare; do you get the error often?
Anyhow, the error is pretty harmless, and it's already on my radar to look into at some point.