Lighthouse & Polymer: start_url in manifest is not cached by Service Worker

I'm testing my Polymer application and I get a pretty good score on Lighthouse. However, I still have a small problem. I have a manifest.json file containing everything so the app can be added to the home screen, but Lighthouse still gives a failure saying: Manifest start_url is not cached by a Service Worker.. I'm using the Polymer Starter Kit 2.0 and I have no idea how to cache it with the Service Worker. I have a sw-precache-config.js containing:

module.exports = {
  staticFileGlobs: [
    '/index.html',
    '/index.html?launcher=true',
    '/manifest.json',
    '/bower_components/webcomponentsjs/*',
  ],
  navigateFallback: '/index.html',
};

Where /index.html?launcher=true is the start_url of the manifest.