org.hibernate.HibernateException: Error while indexing in Hibernate Search (before transaction completion)

I'm receiving the following error while using session.clear() in my batch transaction with hibernate.

org.hibernate.HibernateException: Error while indexing in Hibernate Search (before transaction completion)

I'm using the following code

    try {
        for (TimeSheetEntity timeSheet : timeSheets) {
            timeSheet.setActive(false);
            try {
                session.update(timeSheet);

                count++;

                if (count % 250 == 0 || totalCount == count) {
                    System.out.println(count);

                    session.flush();
                    session.clear();
                }
            } catch (HibernateException ex) {
                logger.error(ArchiveImpl.class.getName(), ExceptionUtils.getStackTrace(ex) + "[" + timeSheet.getId() + "] ");
            } catch (Exception ex) {
                logger.error(ArchiveImpl.class.getName(), ExceptionUtils.getStackTrace(ex) + "[" + timeSheet.getId() + "] ");
            }
        }

        sessionManager.commit();
    } catch (HibernateException ex) {
        System.out.println(ex);
        logger.error(ArchiveImpl.class.getName(), ExceptionUtils.getStackTrace(ex));
        sessionManager.abort();
    } finally {
    }

Now if I remove session.clear(); everything works as it's suppose to. Any ideas why I can't use clear within my batch transaction with hibernate search?