Service Worker TypeError

// JavaScript Document
// Important! Paths are relative to root. Soft link to this file from root to
// avoid scope restrictions.
'use strict;

// Listen for request events
self.addEventListener('fetch', function (event) {

  // Get the request
  let request = event.request;

  // Bug fix
  // https://stackoverflow.com/a/49719964
  if (event.request.cache === 'only-if-cached' && event.request.mode !== 'same-origin') return;

  // HTML files
  // Network-first
  if (request.headers.get('Accept').includes('text/html')) {
    event.respondWith(
      fetch(request).then(function (response) {

        // Create a copy of the response and save it to the cache
        let copy = response.clone();
        event.waitUntil(caches.open('app').then(function (cache) {
          // This is the line that throws an error:
          return cache.put(request, copy);
        }));

        // Return the response
        return response;

      }).catch(function (error) {
        return caches.match(request).then(function (response) {
          return response;
        });
      })
    );
  }

  // CSS, JavaScript, images, & fonts
  // Offline-first
  if (request.headers.get('Accept').includes('text/css') || request.headers.get('Accept').includes('text/javascript') || request.headers.get('Accept').includes('image') || request.headers.get('Accept').includes('font')) {
    // Check the cache first
    // If it's not found, send the request to the network
    event.respondWith(
      caches.match(request).then(function (response) {
        return response || fetch(request).then(function (response) {
          return response;
        });
      })
    );
    return;
  }

});

let coreAssets = [
  // Paths to core assets.
];

// On install, cache some stuff
self.addEventListener('install', function (event) {

  // Cache core assets
  event.waitUntil(caches.open('app').then(function (cache) {
    for (let asset of coreAssets) {
      cache.add(new Request(asset));
    }
    return cache;
  }));
});