Try Core Data with Swift and you will see that happening. Lazy objects (vaults) are mapped from objc into Swift and will happily crash on something like a = b where both are not optional.
This is happening in objc code or in the swift part?
I'm not terribly surprised though, my one experience with core data was miserable once we strayed even a little from the happy path and I ended up rolling my own since we didn't need full functionality anyways. And this was for an internal app, at Apple ┐( ∵ )┌