title: Workbox fallback response with networkFirst strategy
body: <p>I am using Workbox in my service worker and I use <a href="https://developers.google.com/web/tools/workbox/guides/advanced-recipes#provide_a_fallback_response_to_a_route" rel="nofollow noreferrer">this strategy</a> to provide a fallback response to a route that should show when offline and page is not in cache:</p>
  
  <pre><code>const FALLBACK_URL = '/offline/';
  
  const urlHandler = workbox.strategies.staleWhileRevalidate({
      cacheName: 'page-cache'
  });
  
  workbox.routing.registerRoute(
      /\/.+\//,
      ({event}) =&gt; {
          return urlHandler.handle({event})
              .catch(() =&gt; caches.match(FALLBACK_URL));
      });
  </code></pre>
  
  <p>This works well (I have alreaedy cached FALLBACK_URL) when I use staleWhileRevalidate strategy and when I use networkOnly strategy. However, I would really like to use networkFirst strategy but when I try that I get the following error when visiting pages that are not in the cache:</p>
  
  <p><a href="https://i.stack.imgur.com/kY0JS.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/kY0JS.png" alt="enter image description here"></a></p>
  
  <p>and the console says </p>
  
  <p>'The FetchEvent for "<a href="https://staging.bassbuddha.com/pedals/" rel="nofollow noreferrer">https://staging.bassbuddha.com/pedals/</a>" resulted in a network error response: an object that was not a Response was passed to respondWith().'</p>
  
  <p><a href="https://i.stack.imgur.com/eH6Wj.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/eH6Wj.png" alt="enter image description here"></a></p>
  
  <p>What am I doing wrong?</p>
  
  <p>I am using version 3.4.1
  <a href="https://storage.googleapis.com/workbox-cdn/releases/3.4.1/workbox-sw.js" rel="nofollow noreferrer">https://storage.googleapis.com/workbox-cdn/releases/3.4.1/workbox-sw.js</a></p>