SinonJS stub for an async function with one argument and multiple callback values

I have an async function call like this:

db.findUser(username, function (err, user) {

});

I have tried to create the stub like this:

dbMock.findUser = sinon.stub().callsArg(1, "mockUsername", {"err":null, "user":mockFindUserResponse});

Also I tried with callsArgWith:

        dbMock.findUser = sinon.stub().callsArgWith(1, "mockUsername", null, mockFindUserResponse);

But no luck.

Sinon seems to ignore the second callback value.