IXmlSerializable implementation ruins xml serialization

I have a class containing a readonly string. Thus I need to manually handle xmlserialization. But it seems that using this class in a grander setting with automatic xml serialization ruins the deserialization process.

First my readonly class

public class RO : IXmlSerializable
{
  public string Name { get; private set; }

  // for serialization
  protected RO(){}
  public RO(string name)
  {
    this.Name = name;
  }

  public XmlSchema GetSchema()
  {
    return null;
  }

  public void ReadXml(XmlReader reader)
  {
    Name = reader.ReadString();
  }

  public void WriteXml(XmlWriter writer)
  {
    writer.WriteString(Name);
  }
}

then a wrapper class holding my readonly object and some value

public class Holder
{
  public RO a;
  public decimal b;
}

and now I serialize and deserialize an instance of the holder

class Program
{
  static void Main(string[] args)
  {
    XmlSerializer xml = new XmlSerializer(typeof(Holder));
    var s = new StringWriter();
    var holder = new Holder() { a = new RO("foo"), b = 234 };
    xml.Serialize(s, holder);
    string ss = s.ToString();
    Console.WriteLine("*****");
    Console.WriteLine(ss);
    Console.WriteLine("*****");
    holder = (Holder) xml.Deserialize(new StringReader(ss));
    Console.WriteLine(holder.a.Name);
    Console.WriteLine(holder.b);
  }
}

on the screen is a nice-looking xml

*****
<?xml version="1.0" encoding="utf-16"?>
<Holder xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://
 www.w3.org/2001/XMLSchema">
  <a>foo</a>
  <b>234</b>
</Holder>
*****

but when we print the values a and b we get 0 for our decimal.. despite we see the 234 in the xml

foo
0