<bag name="Offices" table="Offices" inverse="true" lazy="true">
    <key column="ParentId"/>
    <many-to-many class="Office" column="Id"/>
</bag>

public void InitProxy(object proxy)
{
    using (ISession session = SessionFactory.OpenSession())
    {
        session.Update(proxy);
        NHibernateUtil.Initialize(proxy);
    }
}

System.InvalidCastException: Unable to cast object of type 'NHibernate.Collection.Generic.PersistentGenericBag`1[IOffice]' to type 'NHibernate.Proxy.INHibernateProxy'.

Server stack trace:
   at NHibernate.Impl.SessionImpl.ReassociateIfUninitializedProxy(Object value)
   at NHibernate.Impl.SessionImpl.Update(Object obj)
   at Manager.InitProxy(Object proxy)

//NHibernateUtil
public static bool IsInitialized(object proxy)
{
      if (proxy is INHibernateProxy)
      {
            return !NHibernateProxyHelper.GetLazyInitializer((INHibernateProxy) proxy).IsUninitialized;
      }
      if (proxy is IPersistentCollection)
      {
            return ((IPersistentCollection) proxy).WasInitialized;
      }
      return true;
}

//SessionImpl
internal bool ReassociateIfUninitializedProxy(object value)
{
      if (!NHibernateUtil.IsInitialized(value))
      {
            INHibernateProxy proxy1 = (INHibernateProxy) value;
            LazyInitializer initializer1 = NHibernateProxyHelper.GetLazyInitializer(proxy1);
            this.ReassociateProxy(initializer1, proxy1);
            return true;
      }
      return false;
}

//SessionImpl
public void Update(object obj)
{
      this.CheckIsOpen();
      if (obj == null)
      {
            throw new ArgumentNullException("obj", "attempted to update null");
      }
      if (!this.ReassociateIfUninitializedProxy(obj))
      {
..skipped
      }
..skipped
}