Почему WCF сериализирует Перечисление как строку?

Код сервера

<DataContract(Namespace:="http://schema.aam.us.com/2010/6", Name:="TradeStatus")>
Public Enum TradeStatus

    NewOrder = 100
    SendToProvider = 101
    ProviderSubmitted = 102
    ProviderAccepted = 103
    ExecutionPending = 104
    Executed = 105
    TicketsCreated = 106 'TERMINAL STATE

End Enum

<DataContract(Namespace:="http://schema.aam.us.com/2010/6", Name:="StatusUpdate")> _
Public Class StatusUpdate

    Public Sub New(ByVal tradeStatus As TradeStatus, ByVal additionalInformation As String)
        Me.TradeStatus = tradeStatus
        Me.AdditionalInforamtion = additionalInformation
    End Sub

    <DataMember(IsRequired:=True)> _
    Public Property AdditionalInforamtion() As String

    <DataMember(IsRequired:=True)> _
    Public Property TradeStatus() As TradeStatus
End Class

Сгенерированный код

<System.Diagnostics.DebuggerStepThroughAttribute(),  _
 System.CodeDom.Compiler.GeneratedCodeAttribute("System.Runtime.Serialization", "4.0.0.0"),  _
 System.Runtime.Serialization.DataContractAttribute(Name:="StatusUpdate", [Namespace]:="http://schema.aam.us.com/2010/6"),  _
 System.SerializableAttribute()>  _
Partial Public Class StatusUpdate
    Inherits Object
    Implements System.Runtime.Serialization.IExtensibleDataObject, System.ComponentModel.INotifyPropertyChanged

    <System.NonSerializedAttribute()>  _
    Private extensionDataField As System.Runtime.Serialization.ExtensionDataObject

    Private AdditionalInforamtionField As String

    Private TradeStatusField As String

    <Global.System.ComponentModel.BrowsableAttribute(false)>  _
    Public Property ExtensionData() As System.Runtime.Serialization.ExtensionDataObject Implements System.Runtime.Serialization.IExtensibleDataObject.ExtensionData
        Get
            Return Me.extensionDataField
        End Get
        Set
            Me.extensionDataField = value
        End Set
    End Property

    <System.Runtime.Serialization.DataMemberAttribute(IsRequired:=true)>  _
    Public Property AdditionalInforamtion() As String
        Get
            Return Me.AdditionalInforamtionField
        End Get
        Set
            If (Object.ReferenceEquals(Me.AdditionalInforamtionField, value) <> true) Then
                Me.AdditionalInforamtionField = value
                Me.RaisePropertyChanged("AdditionalInforamtion")
            End If
        End Set
    End Property

    <System.Runtime.Serialization.DataMemberAttribute(IsRequired:=true, EmitDefaultValue:=false)>  _
    Public Property TradeStatus() As String
        Get
            Return Me.TradeStatusField
        End Get
        Set
            If (Object.ReferenceEquals(Me.TradeStatusField, value) <> true) Then
                Me.TradeStatusField = value
                Me.RaisePropertyChanged("TradeStatus")
            End If
        End Set
    End Property

    Public Event PropertyChanged As System.ComponentModel.PropertyChangedEventHandler Implements System.ComponentModel.INotifyPropertyChanged.PropertyChanged

    Protected Sub RaisePropertyChanged(ByVal propertyName As String)
        Dim propertyChanged As System.ComponentModel.PropertyChangedEventHandler = Me.PropertyChangedEvent
        If (Not (propertyChanged) Is Nothing) Then
            propertyChanged(Me, New System.ComponentModel.PropertyChangedEventArgs(propertyName))
        End If
    End Sub
End Class