如何使用.NET XmlSerializer使值类型为空？

public class Person
{
    public int? Age { get; set; }
}

Person person = new Person();
person.Age = null;

XmlSerializer serializer = new XmlSerializer(typeof(Person));
using (StringWriter writer = new StringWriter())
{
    serializer.Serialize(writer, person);
    string xml = writer.ToString();
}

  <Age xsi:nil="true" />
</Person>