LINQ中的实体附件问题

我试图在从表单帖子收到LINQ实体后将其附加到数据上下文。然而,我得到的只是以下例外:

An entity can only be attached as modified without original state if it declares a version member or does not have an update check policy.

dataContext.People.Attach(person, originalPerson);

在本例中,我得到以下异常:

Object reference not set to an instance of an object.

以下是我的控制器中的代码:

[AcceptVerbs(HttpVerbs.Post)]
public ActionResult Edit(int id, Person person) {
    var prevPerson = dataContext.People.Single(p => p.ID == id);
    dataContext.People.Attach(person, prevPerson);
    dataContext.SubmitChanges();
    return Redirect("~/People/Index");
}

你知道我做错了什么吗?如果需要,我可以发布实体代码。