Erro no hibernate

INFO [STDOUT] 14:58:27,046 ERROR [AssertionFailure] an assertion failure occured (this may indicate a bug in Hibernate, but is more likely due to unsafe use of the session)

org.hibernate.AssertionFailure: null id in br.com.vcn.modelo.CampoDocumento entry (don’t flush the Session after an exception occurs)

alguém tem ideia do que pode ser ?