Is there a trick to creating a custom relationship bit?

Hi! I'm hoping someone here might know the answer to this question - is there a trick to creating a custom relationshipbit? I'm trying to create several for a mod I'm doing, and am able to get the relationship to work during gameplay, but get a last exception and am not able to save afterwards. The error message in the last exception file is about not being able to save due to a value being out of range and specifically references the tuning ID of the new relationshipbit I've added.

The info in the Sim Data file matches the relationshipbits file. I'm sure of that. I've even tried essentially just taking an existing relationshipbit and paired Sim Data file, changing the tuning ID (and instance) for both, and I get the same error.

Is there another file I need to edit or something? This is making me a bit crazy.