Xml serialization of enums - System.InvalidOperationException

A few days ago I was writing a web service when all of a sudden a test failed on me with:

System.InvalidOperationException: Instance validation error: '3' is not a valid value for SomeEnumInMyApp

which turned out to be quite a google-unfriendly exception. The exception is thrown when trying to serialize an enum with a value that isn't explicitly in the declaration of the enum. A couple of people dealt with this, but I don't really want to manually edit the WSDL, and I don't care right now about breaking a future version of the web service (not likely to happen in my case)
In case nothing of this makes any sense to you, let's see some code:
Given this enum declaration:

public enum Epopo {
 Uno = 1,
 Dos = 2
}

"Epopo?" Yes, Epopo. Just because.
This test passes:

[Test]
public void SerializeEnumToXml1() {
 Epopo p = Epopo.Uno;
 using (MemoryStream ms = new MemoryStream()) {
  XmlSerializer xml = new XmlSerializer(typeof(Epopo));
  xml.Serialize(ms, p); 
 }
}

But this one fails with the mentioned exception:

[Test]
public void SerializeEnumToXml2() {
 Epopo p = Epopo.Uno | Epopo.Dos;
 using (MemoryStream ms = new MemoryStream()) {
  XmlSerializer xml = new XmlSerializer(typeof(Epopo));
  xml.Serialize(ms, p); 
 }
}

Hmm. What if we try with a LosFormatter instead of XmlSerializer?

[Test]
public void SerializeEnumToLosFormatter() {
 Epopo p = Epopo.Uno | Epopo.Dos;
 LosFormatter los = new LosFormatter();
 using (MemoryStream ms = new MemoryStream()) {
  los.Serialize(ms, p); 
 }
}

Test passes. Damn you, XmlSerializer. What about XmlEnum?

public enum XmlEpopo {
 [XmlEnum("Uno")]
 Uno = 1,
 [XmlEnum("Dos")]
 Dos = 2
}

[Test] 
public void SerializeEnumWithXmlEnumToXml() { 
 XmlEpopo p = XmlEpopo.Uno | XmlEpopo.Dos; 
 using (MemoryStream ms = new MemoryStream()) { 
  XmlSerializer xml = new XmlSerializer(typeof(XmlEpopo));
  xml.Serialize(ms, p); 
 }
}

Doesn't work either... Well, after that I despaired a bit and started trying everything: XmlElement, PowerCollections.Set, Iesi.Collections.ListSet, Dictionary<Epopo, object>, but nothing worked. Ultimately, I used a List<Epopo>, like this:

[Test]
public void SerializeListOfEnum() { 
 List<Epopo> l = new List<Epopo>();
 l.Add(Epopo.Uno); 
 l.Add(Epopo.Dos); 
 using (MemoryStream ms = new MemoryStream()) { 
  XmlSerializer xml = new XmlSerializer(typeof(List<Epopo>));
  xml.Serialize(ms, l); 
 } 
}

It sucks, I know, but at least it works. If any knows a better solution, feel free to post a comment!
By the way, it seems that Microsoft is having this same problem as well...
Full code for these tests is here.