Testing express routes with mongoose query

I am trying to test my routes that have mongoose queries in. I keep getting back:

AssertionError: expected undefined to equal true

Below is the basic template of my test. At the moment I just want to confirm it calles the res.json.

The route returns all entries in the User model

route.js

const User = require('../../../models/users/users');

const listUsers = (req, res) => {
  User.find((err, users) => {
    if (err) res.send(err);

    res.json(users);
  })
};

module.exports = listUsers;

test.js

const expect = require('chai').expect;
const sinon = require('sinon');

const listUsers = require('../../../../src/routes/api/users/listUsers');

describe('listUsers', () => {
  it('retrieves users', () => {
    const req = {};
    const res = {};
    const spy = res.json = sinon.spy;

    listUsers(req, res);
    expect(spy.calledOnce).to.equal(true);
  })
});

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