How to resolve "Cannot read property 'should' of undefined" in chai?

I'm trying to test my test my RESTful nodejs API but keep running into the following error.

Uncaught TypeError: Cannot read property 'should' of undefined

I'm using the restify framework for my API.

'use strict';

const mongoose = require('mongoose');
const Customer = require('../src/models/customerSchema');

const chai = require('chai');
const chaiHttp = require('chai-http');
const server = require('../src/app');
const should = chai.should();

chai.use(chaiHttp);

describe('Customers', () => {
   describe('/getCustomers', () => {
       it('it should GET all the customers', (done) => {
           chai.request(server)
               .get('/getCustomers')
               .end((err, res) => {
                   res.should.have.status(200);
                   res.body.should.be.a('array');
                   done();
                });
       });
   });
});

The testing works fine when I remove the line res.body.should.be.a('array'); Is there anyway I can fix this problem?