Using compound primary key throws SQLException

Using compound primary keys via @IdClass causes an exception when calling persist:

Caused by: javax.persistence.PersistenceException: org.hibernate.exception.GenericJDBCException: could not insert: [de.methodpark.stages.model.UserAssignment]
at org.hibernate.ejb.AbstractEntityManagerImpl.convert(AbstractEntityManagerImpl.java:1214)
at org.hibernate.ejb.AbstractEntityManagerImpl.convert(AbstractEntityManagerImpl.java:1147)
at org.hibernate.ejb.AbstractEntityManagerImpl.convert(AbstractEntityManagerImpl.java:1153)
at org.hibernate.ejb.AbstractEntityManagerImpl.persist(AbstractEntityManagerImpl.java:678)
at com.sun.enterprise.container.common.impl.EntityManagerWrapper.persist(EntityManagerWrapper.java:258)

Same code works when switching JPA provider to EclipseLink.

@Entity
@Table(name = "user_assignment")
@IdClass(value = UserAssignmentId.class)
public class UserAssignment implements Serializable {
@Id
private Long userId;

@Id
private Long elementId;

@ManyToOne(fetch = FetchType.LAZY, optional = false)
@JoinColumn(name = "element_id", referencedColumnName = "id")
private Element element;

@ManyToOne(fetch = FetchType.LAZY, optional = false)
@JoinColumn(name = "user_id", referencedColumnName = "id")
private User user;
}

public class UserAssignmentId implements Serializable {

public UserAssignmentId() { }

@Column(name = "user_id", insertable = false, updatable = false)
public Long userId;

@Column(name = "element_id", insertable = false, updatable = false)
public Long elementId;
}