MethodMissingException?

I have code that is similar to the following:

User user = new User();
Ticket ticket = new Ticket();

//Setup mocks
MockManager.Init();
MockObject<TicketManager> mockTicketManager = MockManager.MockObject<TicketManager>(Constructor.Mocked);
mockTicketManager.ExpectAndReturn("Save", ticket);
mockTicketManager.ExpectUnmockedCall("AddSubscriber");

TicketManager ticketManager = mockTicketManager.Object;
SubscriberRelationship relationship = ticketManager.AddSubscriber(user, ticket);

When I run this test under the debugger (VSTS w/ MSTest) it works fine. When run it normally I get a runtime exception "System.MissingMethodException: Method not found: 'Void TicketManager.AddSubscriber(User, Ticket)'

I'm pretty sure I'm doing something wrong, or at least unexpected. Any ideas?