Two way function does not return data with custom data types

I have been trying to get started with WCF and have hit a wall. I have a
self-hosted WCF service that uses dupex netTcpBinding to allow two-way data
flow. So far it works great with sending basic data types as a return value
when I call a method from a client, but when I return a custom class or
structure, its data members always come back as nothing. Any ideas? Here is a
sample of my implementation:

<ServiceContract(SessionMode:=SessionMode.Required,
CallbackContract:=GetType(IServerCallback))> _
Interface IServer

<OperationContract(IsOneWay:=False, IsInitiating:=True,
IsTerminating:=False)> _
Function Login() As LoginResult

End Interface

<DataContract()> _
Public Class LoginResult

<DataMember()> _
Public Value As String

End Class

<ServiceBehavior(ConfigurationName:="GameStreet.GSTalkServer", _
InstanceContextMode:=InstanceContextMode.PerSession, _
ConcurrencyMode:=ConcurrencyMode.Multiple)> _
Public Class MyService
Implements IServer

Public Function Login() As LoginResult Implements IServer.Login
Dim oRes As New LoginResult
oRes.Value = "Any value..."
Return oRes
End Function

End Class