Fluent nHibernate映射问题

我有这些表/类（示例）：

table FirstTable (
    Id INTEGER NOT NULL DEFAULT AUTOINCREMENT, 
    Name VARCHAR(100) NOT NULL,
    Document VARCHAR(20) NOT NULL
)
table SecondTable (
    Id INTEGER NOT NULL,
    Something VARCHAR(100) NULL,
    FOREIGN KEY (Id) REFERENCES FirstTable (Id)
)
public class FirstClass {
    public string Name { get; set; }
    public string Document { get; set; }
    public SecondClass SecondClass { get; set; }
}
public class SecondClass {
    public string Something { get; set; }
    public FirstClass FirstClass { get; set; }
}

映射为：

public class FirstClassMap : ClassMap<FirtsClass> {
    Table("FirstTable");
    Id(x => x.Id).GeneratedBy.Identity();
    Map(x => x.Name);
    Map(x => x.Document);
    References(x => x.SecondClass, "Id").ForeignKey();
}
public class SecondClassMap : ClassMap<SecondClass> {
    Table("SecondTable");
    Id(x => x.Id).GeneratedBy.Foreign("FirstClass");
    Max(x => x.Something);
    HasOne(x => x.FirstClass).PropertyRef(x => x.SecondClass).Cascade.SaveUpdate();
}

FirstClass可以有（0,1）SecondClass，SecondClass可以有一个（1,1）FirstClass。

以下代码返回错误"尝试从空的一对一属性分配id：SecondClass"

var test = new SecondClass();
test.FirstClass = new FirstClass();
test.Something = "New test";
test.FirstClass.Name = "My name";
test.FirstClass.Document = "My document";
// ... commands to save.