Hi Matt
unfortunately I haven't been able to re-create this issue systematically. I can reproduce it accidently plenty of times, but for example I can apply a rule to a new parcel, and get the error, then delete the rule and re apply if to the same parcel, it applys fine half the time, and the other half it causes my model to go crazy...
I just got the same error today when I was modifying a rule by changing an attr to a const. In this case I hadn't saved before re-generating the model which again makes me think that saving the rule is linked to it in some way but not necessarily the whole issue.
I generally get this issue happening at least twice a day for various reasons...
I'll let you know if I come up with anything else.
Regards
Joanne