As seen in HHH-9165 Resolved , and increasingly in StackOverflow questions like https://stackoverflow.com/q/52108442/ where people want to use the upsert syntax, and with my own experience this is a problem with the database itself, but I believe I found a way it can be addressed by setting the expected type of a parameter before setting it as null (or an unregistered subclass extending a registered one). I have a working example for a PR.