Mocking repository and callback issue

I have a Repository which I have mocked using Moq.

I have a Service which I have mocked. The service expects a repository in its constructor and I have successfully been able to do this.

The problem I am now facing is that, I want the Repository to not connect to the DB but instead use an in memory storage to query/perform operations against.

if I do this:

userMockRepository.Setup(r => r.Login(It.IsAny<string>(), It.IsAny<string>()))
                  .Returns((User u) => users.Find(p => p.Password == u.Password &&
                                                       p.Username == u.Username));

then run the test, I get an exception:

Parameter count mismatch

when the Service method is being called when about to invoke the Login on the UserRepository.

What am I doing wrong? Am I misunderstanding?

Essentially any Repository calls should be redirected or query against an in memory collection (i.e List)

this is my actual service layer method:

var user = uoWContext.UserRepository.Login(request.Username, request.Password);

if (user == null)
{
    response.Success = false;
    response.FailureInformation = "Invalid username or password";
}

return response;

When it gets to the line uowContext.UserRepository.Login(...) - this is where I get the exception.