Hibernate envers, post-delete event throws constraint violation exception

Project uses Hibernate 3.5, Spring Webflow 2 and Hibernate Envers for auditing. Envers is configured in hibernate.cfg.xml. I have one-to-many relation mapping between entity 'ArticleGroup' and 'Article'. Table 'articles' has a foreign key 'article_group_id' references to the id in table 'article_groups'. In the front-end, when I delete an article, Hibernate Envers throws constraint violation exception for the post-delete event. If I don't use Envers, delete operations work fine. The two entities are defined as follows

    @Entity
    @Table(name="article_groups")
    @Audited
    public class ArticleGroup implements Serializable {

        @OneToMany(mappedBy="articleGroup", fetch = FetchType.EAGER, cascade = CascadeType.ALL)
        @OrderBy("id")
        private List<Article> articles;

        // other fields, getters and setters
    }

    @Entity 
    @Table(name="articles")
    @Audited
    public class Article implements Serializable {

        @ManyToOne
        @JoinColumn(name = "article_group_id")
        private ArticleGroup articleGroup;

        // other fields, getters, setters
    }

Article deletion is coded as follows:

    @Service("articleManager")
    public class ArticleManagerImpl implements ArticleManager {
            // inject dao
            @SuppressWarnings("unchecked")
            @Transactional(readOnly = false, propagation = Propagation.REQUIRES_NEW)
            public void deleteArticle(Article article, Object articles) {
            articleDao.delete(article);
            ((List<Article>)  ((OneSelectionTrackingListDataModel)articles).getWrappedData()).remove(article);
        }
    }

The constraint violation exception:

Hibernate: delete from live.articles where id=?
Hibernate: select nextval ('hibernate_sequence')
Hibernate: insert into audit.REVINFO (REVTSTMP, REV) values (?, ?)
Hibernate: insert into audit.articles_AUD (REVTYPE, content, language, name, order_number, title, article_group_id, id, REV) values (?, ?, ?, ?, ?, ?, ?, ?, ?)
82828 [http-bio-8080-exec-2] DEBUG   org.springframework.orm.hibernate3.HibernateTransactionManager  - Initiating transaction rollback after commit exception
org.springframework.dao.DataIntegrityViolationException: Could not execute JDBC batch update; nested exception is org.hibernate.exception.ConstraintViolationException: Could not execute JDBC batch update
...
Caused by: java.sql.BatchUpdateException: Batch entry 0 insert into audit.articles_AUD (REVTYPE, content, language, name, order_number, title, article_group_id, id, REV) values ('2', NULL, NULL, NULL, NULL, NULL, NULL, '14', '17') was aborted.  Call getNextException to see the cause.
at org.postgresql.jdbc2.AbstractJdbc2Statement$BatchResultHandler.handleError(AbstractJdbc2Statement.java:2619)

As you could see, when Envers inserts into table 'audit.articles_AUD', the article_group_id is null which caused the constraint violation. Anybody knows how to fix it? Thank you very much.