supertest as promised wtf.js

'use strict';

var supertest = require('supertest-as-promised');
var express = require('express');


const app = express();

function fetch() {
  return supertest(app).get('/')
    .send();
}

Promise.resolve(1)
  .then((x) => {
    var res = fetch();
    console.log('First', typeof res.expect); // this is Function as expected.
    return res;
  })
  .then((res) => {
    console.log('Second', typeof res.expect); // why is this undefined?
    return res;
  });