执行添加功能

private void buttonAdd_Click(object sender, EventArgs e)
        {
            Customer customer = new Customer();
            customer.CustomerId =Int32.Parse(this.textBoxCustomerId.Text);
            customer.FirstName = this.textBoxFirstName.Text;
            customer.LastName = this.textBoxLastName.Text;
            customer.Version =Int32.Parse( this.textBoxVersion.Text);
            reference.Add(customer);

            comboBox.Items.Add(this.textBoxCustomerId.Text);
        }

时出现could not insert: [fuck.Model.Customer][SQL: INSERT INTO Customer (FirstName, LastName, Version) VALUES (?, ?, ?); select SCOPE_IDENTITY()]

"不能将值 NULL 插入列 'CustomerId'，表 'NHibernateSample.dbo.Customer'；列不允许有空值。INSERT 失败。\r\n语句已终止。"}

持久类是public class Customer
    {
        public virtual int CustomerId { get; set; }
        public virtual string FirstName { get; set; }
        public virtual string LastName { get; set; }
        public virtual int Version { get; set; }
        public virtual ISet<Order> Order { get; set; }

}

映射文件是

<?xml version="1.0" encoding="utf-8" ?>
<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2">
  <class name="fuck.Model.Customer,fuck.Model" table="Customer">
    <id name="CustomerId" type="Int32" unsaved-value="null">
      <column name="CustomerId" length="4" sql-type="int" not-null="true" unique="true" index="PK_Customer"/>
      <generator class="native" />
    </id>
    <property name="FirstName" type="String">
      <column name="FirstName" length="50" sql-type="varchar" not-null="true"/>
    </property>
    <property name="LastName" type="String">
      <column name="LastName" length="50" sql-type="varchar" not-null="true"/>
    </property>
    <property name="Version" type="int">
      <column name="Version" length="4" sql-type="int" not-null="true"/>
    </property>

    <!--一对多关系：Customer有一个或多个Orders-->
    <set name="Order" table="Order" generic="true" inverse="true">
      <key column="CustomerId" foreign-key="FK_Order_Customer"/>
      <one-to-many class="fuck.Model.Order,fuck.Model"/>
    </set>
  </class>
</hibernate-mapping>