entity is not associated with the session: null

Caused by:org.hibernate.HibernateException: entity is not associated with the session: null
    org.hibernate.persister.entity.AbstractEntityPersister.initializeLazyProperty(AbstractEntityPersister.java:735)
    org.hibernate.intercept.AbstractFieldInterceptor.intercept(AbstractFieldInterceptor.java:73)
    org.hibernate.intercept.cglib.FieldInterceptorImpl.readObject(FieldInterceptorImpl.java:75)
    报错如上，请问是什么原因导致的？