When unit testing with Chai, what does "TypeError: Cannot read property 'address' of undefined" mean?

I keep getting this error message when unit testing using chai and none of the tests passes even though they should. What does it mean in this context? Thanks.

var chai = require('chai');
var chaiHttp = require('chai-http');
var server = require('../server.js');

var should = chai.should();
var app = server.app;
var storage = server.storage;

chai.use(chaiHttp);

describe('Shopping List', function() {
  it('should list items on GET', function(done) {
chai.request(app)
  .get('/items')
  .end(function(err, res) {
    res.should.have.status(200);
    res.should.be.json; // jshint ignore:line
    res.body.should.be.a('array');
    res.body.should.have.length(3);
    res.body[0].should.be.a('object');
    res.body[0].should.have.property('id');
    res.body[0].should.have.property('name');
    res.body[0].id.should.be.a('number');
    res.body[0].name.should.be.a('string');
    res.body[0].name.should.equal('Broad beans');
    res.body[1].name.should.equal('Tomatoes');
    res.body[2].name.should.equal('Peppers');
    done();
  });
   });