nhibernate set null to DateTime column

1)

[DataMember]
public virtual Nullable<DateTime> DelayedDeliveryDate
{
　　get;
　　set;
}

2)

if (order.DelayedDeliveryDateForSaveOrUpdate == "")
{
　　createOTCOrder.SetParameter("DelayedDeliveryDate", null, NHibernateUtil.DateTime);
}
else
{
　　createOTCOrder.SetDateTime("DelayedDeliveryDate", Convert.ToDateTime(order.DelayedDeliveryDateForSaveOrUpdate));
}