Sinon anonymous stub passed as a parameter

I am trying to test a function in a redux container but the issue is barely about redux or react. Basically the fetchData function I am trying to test, takes two functions as parameters and calls them.

What I was hoping to do was to have two anonymous stubs and pass them to the function this way:

var firstStub = sinon.stub().withArgs(mockResponse).returns('success');
var secondStub = sinon.stub().withArgs(mockResponse).returns('success');
AccountApp.fetchData({ firstStub , secondStub });

When this happens my function fetchData complains about the firstStub and secondStub not being a function. I know they are stub objects but if that is the case what is the correct way of managing this situation.