MidiClient.CreateVirtualSource() returns null

I'm trying to create a virtual endpoint within my application, but the CreateVirtualSource method keeps returning null, not a MidiEndpoint... Here is my relevant code:

var _client = new MidiClient("My MIDI Client");
var _endpoint = _client.CreateVirtualSource("foo");

_endpoint is always null. Does anyone know how to make this work?

Thanks!

Josh