WCF wsDualHttpBinding callback

I mean the Server side cannot send result to client side...

SERVER:
public void GetData(ObservableCollection<People> listValues)
{
Callback.ResultData(listValues);
}

IDuplexCallback Callback
{
get
{
return OperationContext.Current.GetCallbackChannel<iduplexcallback>();
}
}


CLIENT:

// Construct InstanceContext to handle messages on callback interface.
InstanceContext instanceContext = new InstanceContext(new CallbackHandler());

// Create a client.
DuplexServiceReference.DuplexClient client = new DuplexServiceReference.DuplexClient(instanceContext);

client.GetData(observableCollectionPeople);


public class CallbackHandler : IDuplexCallback
{
public void ResultData(ObservableCollection<People> listInt)
{
System.Windows.MessageBox.Show(listInt.Count().ToString());
}
}


When debugging, I enter in the server code. When the server (service) code try to call back the client (Callback.ResultData) it fails with timeout exception.

But if i instead of ObservableCollection I use an integer, it works.