NHibernate映射索引超出范围

我有两个类:

 public class CarModel
    {
        public virtual int Id { get; set; }
        public virtual string model_name { get; set; }
    }

和

  public class Transport
    {
          public virtual int Id { get; set; } 
          public virtual string lic_plate { get; set; } 
          public virtual int model { get; set; } 
          public virtual string odometer { get; set; }
          public virtual string moto_val { get; set; } 
          public virtual Class.CarModel Modelis { get; set; }    
    }

和映射:

<?xml version="1.0" encoding="utf-8" ?>
<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2" auto-import="true" assembly="web_nt" namespace="web_nt.Models">
  <class name="Transport" table="transport" dynamic-update="true" lazy="false">
    <id name="Id" column="Id" type="int">
      <generator class="native" />
    </id>
    <property name="lic_plate" />
    <property name="model" />
    <property name="odometer" />
    <property name="moto_val" />
    <many-to-one name="Modelis" column="model"  cascade="none" class="web_nt.Models.Class.CarModel" lazy="false" /> 
  </class>
  <class name="web_nt.Models.Class.CarModel" table="car_model">
    <id name="Id" column="id" type="int">
      <generator class="native" />
    </id>
    <property name="model_name" />
  </class>
</hibernate-mapping>

当我尝试将值发送到数据库时(在视图中它完美地工作):+ $exception{"索引超出范围。必须非负且小于集合的大小。'r'nParameter name: index"}系统。异常{系统。ArgumentOutOfRangeException}

我找不到这里可能有什么问题?

<property name="model" />    
<many-to-one name="Modelis" column="model"  
     cascade="none" class="web_nt.Models.Class.CarModel" lazy="false" /> 

<property name="model" insert="false" update="false" />    
<many-to-one name="Modelis" column="model"  
     cascade="none" class="web_nt.Models.Class.CarModel" lazy="false" />