Hi,

This is a change I detected during hibernate 4 to 5 migration. Using null values as natural-id does not throw exception anymore.

This may be a change in hibernate 5, but the documentation still says that it should not be possible: https://docs.jboss.org/hibernate/orm/5.0/manual/en-US/html/ch05.html#mapping-declaration-naturalid

A natural key is a property or combination of properties that is unique and non-null.