Nightmare inside then cant use pdf

the code:

var Nightmare = require('nightmare');
var nightmare = Nightmare({show:true});

nightmare
  .goto('https://duckduckgo.com')
  .evaluate(() => { return document.title; })
  .end()
  .then((title) => {
    console.log(title);
    nightmare.pdf(title, {printBackground: true});
  });

Why it isn't saving the pdf with "title"? And even if I change the title to some string like pdf.pdf it still doesn't work inside .then() why is that?

I have found out why it was happening:

The .end() was causing the other functions not to work inside .then, after moving it inside .then it started working as needed. Code:

nightmare
  .goto('https://duckduckgo.com')
  .evaluate(() => { return document.title; })
  //.end() // <= this one is bad
  .then(function (title) {
    console.log(title);
    nightmare.end() // <= This one is good.
    return nightmare.pdf(title, {printBackground: true});
  })