"default/aisettler" sanity checks

[FriendAtArms]

It's been a couple of months, and my ruleset is chugging along as I find time for it. Every couple of test games, this message appears about Turn 20 or so:

in cityresult_fill() [../../../../../ai/default/aisettler.c::452]: assertion 'result->remaining >= 0' failed.

This isn't the first time I've had a "default/aisettler" problem, but it is the first time it's happened in v3.0. And it doesn't happen with every test. Can anyone figure out what's gone wrong?