This goes to the feature we just added in 5.0 of supporting "non standard" types as identifiers. Specifically UUID. Sanne and I have been discussing the MySQL (MariaDB) tests hanging recently. Well on the ORM side this was caused by an overly optimistic tests not properly handling transactions and connections on an exception. The underlying exception however comes from this support running against MySQL. The UUID is treated as BINARY. The test inserts a row, and then deletes it by id. The row is inserted no problem. However the delete by id fails. Apparently MySQL is not liking comparisons using = based on binary data. The work around is to explicitly add @Column( length = 16 ). This is planned to be addressed later via org.hibernate.type.Type#dictatedSizes Just a heads up if ya'll run into this. I added a note to the migration guide as well.