Почему NHibernate выдает «StaleObjectStateException»?

Я пишу проект и использую NHibernate 3.1

SimpleTest:

Forum forum = Session.CreateCriteria<Forum>().Add(Restrictions.Eq("UrlName", "reportabug")).UniqueResult<Forum>();
forum.TopicsCount++;
IForumRepository forumRepository = new ForumRepository(SessionFactory);
forumRepository.Update(forum);

public virtual void Update(TEntity entity)
{
    if (!session.Transaction.IsActive)
    {
        TResult result;
        using (var tx = session.BeginTransaction())
        {
            session.SaveOrUpdate(entity)
            tx.Commit();
        }
        return result;
    }
    session.SaveOrUpdate(entity)
}

Последнее обновление выдает исключение:

StaleObjectStateException was unhandled by user code:
    Row was updated or deleted by another transaction (or unsaved-value mapping was incorrect)

SQL-запрос:

UPDATE Forums
SET    Name = 'Forums Issues (not product support)' /* @p0 */,
       UrlName = 'reportabug' /* @p1 */,
       Description = 'Use this forum to report issues with the online forums application. When reporting an issue please include relevant details such as repro steps, error messages and browser version.' /* @p2 */,
       CategoryId = 'b2cc232c-0d5c-4f35-bb6f-29c67d7d40c2' /* @p3 */,
       TopicsCount = 1 /* @p4 */
WHERE  ForumId = '864046b7-ca57-48c4-8a81-082103223527' /* @p5 */

ForumId правильный. Может быть, это параллелизм? Есть идеи?