Sinon Spy not being called with right arguments

Background

I am trying to learn how to do a RESTful API following the TDD paradigm by reading a book on the subject (it is in brazillian):

• https://leanpub.com/construindo-apis-testaveis-com-nodejs/read

The author encourages the use a sinon.js together with mocha.js.

I am getting close to the end, but I am failling to pass the test for my gnomeController.

Problem

The problem is that I am using sinon to assert that I am calling the gnomeController's get method using the given reponse object, which is in reallity a spy.

This spy is to make sure I call the reponse method with an "Error", but it appears I am calling the response with no arguments whatsoever, which is very confusing.

Code

gnomeController.js

module.exports = aGnomeModel => {

    let Gnome = aGnomeModel;

    function get(req, res){
        return Gnome.find({})
            .then(gnomes => res.send(gnomes))
            .catch(err => res.status(400).send(err));
    }

    return Object.freeze({
        get
    });
};

gnomeTest.js

const sinon = require("sinon");
const gnomesControllerFactory = require("gnomesController.js");
const Gnome = require("gnomeModel.js");

describe("Controllers: Gnomes", () => {

    describe("get() gnomes", () => {

        it("should return 400 when an error occurs", () => {
            const request = {};
            const response = {
                send: sinon.spy(),
                status: sinon.stub()
            };

            response.status.withArgs(400).returns(response);
            Gnome.find = sinon.stub();
            Gnome.find.withArgs({}).rejects("Error");

            const gnomesController = gnomesControllerFactory(Gnome);

            return gnomesController.get(request, response)
                .then(arg => {
                    console.log(arg);
                    sinon.assert.calledWith(response.send, "Error");
                });
        });
    });

});

Question

I am using the latest versions of both libraries.

1. What is wrong in my code, why is the reponse being called with no arguments?

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