Google App Engine - org.datanucleus.exceptions.NucleusUserException: Object Manager has been closed

Question

Why does the code below result in org.datanucleus.exceptions.NucleusUserException: Object Manager has been closed? The exception appears to be thrown at query.getResultList(

Answer 1

Adding @Transactional to the method prevents the object manager from closing. However, I'm not sure why it closes without this.