This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
That isn't an exception. Is there a stack trace? In the table in the database how is the primary key getting set. With just @Id, it is your apps responsibility to set the id. Are you using a Sequence table?