请教一个Hibernate的AssertionFailure问题

出现
2006-08-31 13:19:24 ERROR AssertionFailure:22 - 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 common.databinding.NtStateChange entry (don't flush the Session after an exception occurs)
错误，请问是怎么回事？

Hibernate版本是3.1.3，数据库是Sybase，谢谢了！

请大家帮忙看看阿，