列挙型の変数を持ったクラスをシリアライズするとエラー？

Imports System.IO

Imports System.Xml.Serialization



Namespace IO

    ''' <summary>

    ''' オブジェクトをシリアライズしてXMLファイルに出力したり、XMLファイルをデシリアライズしてオブジェクトを生成したりするクラス

    ''' </summary>

    ''' <typeparam name="T">オブジェクトの型</typeparam>

    ''' <remarks></remarks>

    Public Class ObjectSerializer(Of T)

        Private mFileName As String

        Private mPath As String = Application.StartupPath



        ''' <summary>

        ''' XMLファイル名

        ''' </summary>

        ''' <param name="fileName"></param>

        ''' <remarks></remarks>

        Public Sub New(ByVal fileName As String)

            mFileName = fileName

        End Sub



        Property Path() As String

            Get

                Return mPath

            End Get

            Set(ByVal value As String)

                mPath = value

            End Set

        End Property



        ''' <summary>

        ''' オブジェクトをシリアライズしてXMLファイルに出力する

        ''' </summary>

        ''' <param name="obj"></param>

        ''' <remarks></remarks>

        Public Sub Serialize(ByVal obj As T)



            Dim serializer As XmlSerializer = New XmlSerializer(GetType(T))

            Dim stream As FileStream = New FileStream(mPath & mFileName, FileMode.Create)

            Try

                serializer.Serialize(stream, obj)

            Finally

                stream.Close()

            End Try



        End Sub



        ''' <summary>

        ''' XMLファイルをデシリアライズしてオブジェクトを生成する

        ''' </summary>

        ''' <returns></returns>

        ''' <remarks></remarks>

        Public Function Deserialize() As T

            Dim path As String = Application.StartupPath & mFileName

            If File.Exists(path) Then

                Dim serializer As XmlSerializer = New XmlSerializer(GetType(T))

                Dim stream As FileStream = New FileStream(path, FileMode.Open)

                Try

                    Return CType(serializer.Deserialize(stream), T)

                Finally

                    stream.Close()

                End Try

            Else

                Return Nothing

            End If

        End Function



    End Class

End Namespace

Public Class XmlTest1

    Public a As Integer

    Public b As New XmlTest2

End Class



Public Class XmlTest2

    Public b As Integer

    Public c As Types = Types.aa

    'Public d As EnumTest.Types = EnumTest.Types.cc '<=こいつを足すとエラー

    Enum Types

        aa

        bb

    End Enum

End Class



Public Class EnumTest

    Enum Types

        cc

        dd

    End Enum

End Class

    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click

        Dim xml As New ObjectSerializer(Of XmlTest1)("\aaa.xml")

        Dim obj As New XmlTest1



        xml.Serialize(obj)

    End Sub

End Class