How to insert value in foreign key column, reference is on same table's primary key using JPA

Question

I need to insert a null value in foreign key column, but I get this error: Is it possible?

Exception

org.springframework.dao.InvalidDataAccessApiUsageException: org.hibernate.TransientPropertyValueException: object references an unsaved transient instance - save the transient instance before flushing: com.company.project.model.ConceptoGasto.conceptoGasto -> com.company.project.model.ConceptoGasto; nested exception is java.lang.IllegalStateException: org.hibernate.TransientPropertyValueException: object references an unsaved transient instance - save the transient instance before flushing: com.company.project.model.ConceptoGasto.conceptoGasto -> com.company.project.model.ConceptoGasto

Entity

//----------------------------------------------------------------------
// ENTITY PRIMARY KEY ( BASED ON A SINGLE FIELD )
//----------------------------------------------------------------------
@Id
@GeneratedValue(strategy=GenerationType.AUTO)
@Column(name="id", nullable=false)
private Integer id;

....

@ManyToOne (optional = true)
@JoinColumn(name="idPadre", referencedColumnName="id", nullable=true)
private ConceptoGasto conceptoGasto;

.....

Method

JpaOperation operation = new JpaOperation() {
 @Override
 public Object execute(EntityManager em) throws PersistenceException {
   T managedEntity = em.merge(entityToSave);
   //em.refresh(managedEntity);
   return managedEntity;
 }
 } ;