问为什么我在XML反序列化函数中得到这个错误？

EN

public string Serialize(Object o)
{
    using (var writer = new StringWriter())
    {
        new XmlSerializer(o.GetType()).Serialize(writer, o);
        return writer.ToString();
    }
}

public PathDetailsMessage Deserialize(string xml)
{
    using (TextReader reader = new StringReader(xml))
    {
         XmlSerializer serializer = new XmlSerializer(typeof(PathDetailsMessage));
         return (PathDetailsMessage)serializer.Deserialize(reader);
    }
}

static void Main(string[] args)
{
    PathDetailsBLL train = new PathDetailsBLL();
    PathDetailsMessage pdm = train.GetDetails();
    string xml = train.Serialize(pdm);
    PathDetailsBLL dsa = new PathDetailsBLL();
    PathDetailsMessage fds = new PathDetailsMessage();
    fds = dsa.Deserialize(pdm.ToString());
    Console.Write(fds);
    Console.ReadKey();
}