Angular testing, mocking rootscope

I am trying to mock the rootScope.$id property and I am doing something incorrectly that I can't quite put my finger on. So I am using karma/jasmine and sinon for the stubbing . Here's what I have -

uptop - defining the mock

   var mockRootScope = {
            $id: sinon.stub()
        };

Then in the before each

   angularMocks.module(function($provide) {
                    $provide.value('$rootScope', mockRootScope);
                });

And trying to set it in the unit-test like so

   mockRootScope.returns({
                    $id: '1'
                });

Doesn't seem to be mocking it up correctly - Any idea what I'm doing wrong here? Thanks!