LINQ InsertOnSubmit:NullReferenceException

using System;
using System.Linq;
using System.Data.Linq;

public class Customer
{
    public int ID { get; set; }
    public string Name { get; set; }
}

public class Program
{
    public static void Main()
    {
        DataContext db = new DataContext("connectionString");

        // 创建一个新的 Customer 对象
        Customer newCustomer = new Customer { ID = 1, Name = "John Doe" };

        // 将新的 Customer 对象插入到数据库中
        db.Customers.InsertOnSubmit(newCustomer);

        // 提交更改
        db.SubmitChanges();
    }
}