HashTable Serialazation

Does anyone know why I can serialize a Hashtable to a binary format but It wont serialize to a XML format?

Code:

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Imports System
Imports System.Xml
Imports System.Xml.Serialization
Imports System.IO

Public Class Form1
    Inherits System.Windows.Forms.Form

    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
        Dim aCnt As Int16
        Dim TestHash As New Hashtable()

        'Load up some dummy classes into the ht
        For aCnt = 1 To 20
            Dim TestCls As New clsTest("MyName" & aCnt.ToString, aCnt)
            TestHash.Add(aCnt, TestCls)
        Next

        'Save it to disk as binary (works fine)
        If File.Exists("hash.dat") = True Then File.Delete("hash.dat")
        Dim fs As New FileStream("hash.dat", FileMode.CreateNew)
        Dim bf As New Runtime.Serialization.Formatters.Binary.BinaryFormatter()
        bf.Serialize(fs, TestHash)
        fs.Close()

        'Save it to disk in XML (doesn't work)
        If File.Exists("hash.xml") = True Then File.Delete("hash.xml")
        Dim fsx As New FileStream("hash.xml", FileMode.CreateNew)
        Dim bfx As New XmlSerializer(GetType(clsTest))
        bfx.Serialize(fsx, TestHash)
        fsx.Close()

    End Sub

End Class

<Serializable()> Public Class clsTest

    Public sName As String
    Public MyID As Int16

    Public Sub New()
        'Default constructor
    End Sub

    Public Sub New(ByVal iName As String, ByVal iID As Int16)
        sName = iName
        MyID = iID
    End Sub

End Class

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