Why I could not serialize a tuple in C#?

Possible Duplicate:
Why XML-Serializable class need a parameterless constructor

I'm trying to serialize a tuple in my code:

List<List<Tuple<String, CodeExtractor.StatementNode>>> results = null;
results = extractor.ExtractSourceCode(sourceCode);
FileStream fs = new FileStream(@"C:\Projects\Test\ast.xml", FileMode.Create);

XmlSerializer formatter = new XmlSerializer(
    typeof(List<List<Tuple<String, CodeExtractor.StatementNode>>>));

formatter.Serialize(fs, results);

fs.Close();

but it was failed and catch the exception like this:

System.Tuple`2[System.String,CodeExtractor.StatementNode] cannot be serialized because it does not have a parameterless constructor.

and I'm do sure the CodeExtractor.StatementNode could be serialized.