c# - instantiating an nhibernate Collection -

in order entity getting null ref on orderitems. occurs when order first created. i'd able add like

order.orderitems = new iesi.collections.set<orderitem>(); 

the error "cannot access protected constructor "set" here

    public class order     {         public virtual int id { get; set; }         public virtual iset<orderitem> orderitems { get; set; }          public virtual void addorderitem(orderitem item)         {             item.order = this;             // orderitems below null             orderitems.add(item);          }      }   <?xml version="1.0" encoding="utf-8" ?> <hibernate-mapping xmlns="urn:nhibernate-mapping-2.2"                    assembly="estore.domain"                    namespace="estore.domain.model">    <class name="order" table="orders">     <id name="id" type="int32" column="id" unsaved-value="0">       <generator class="identity"/>     </id>      <property name="username" column="username"/>     <set name="orderitems" table="orderitems" generic="true" cascade="all-delete-orphan"  inverse="true" >       <key column="orderid" not-null="true" />       <one-to-many class="estore.domain.model.orderitem, estore.domain" />     </set>    </class> </hibernate-mapping> 

set orderitems new list in constructor

public order() {  orderitems = new list<orderitem>(); }