[dolmen]

But first ensure that the input error is properly reported to the client in the response body (ideally in a structured way), so the client could have figured out by himself.

If a fix is needed on your side for this matter, having a conversation with a customer might be useful before breaking more stuff. ("We have no state code in EU. Why is that mandatory?").

[lanstin]

If you are trying to sell a product, it is sometimes useful to solve people problems for them, rather than counting on them to figure them out on their own.