The API in question is almost certainly internal. The only reason it isn’t marked as such is because Python doesn’t have great facilities for that kind of encapsulation.

Invariant-preserving types are always going to be the right way to eliminate certain classes of bugs, but they’re also completely overkill in this context given that the “bug” in question doesn’t even cause unsound program behavior; it just raises an exception, which is completely sound and well-defined.