title: I get an error when I use BroadcastUpdatePlugin (workbox-broadcast-update)
body: <p>I want to use BroadcastUpdatePlugin (workbox-broadcast-update) in my application, but I have an error something like this:</p>
  
  <p><a href="https://i.stack.imgur.com/Jqnfn.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/Jqnfn.png" alt="enter image description here"></a></p>
  
  <p>My route:</p>
  
  <pre><code>registerRoute(
          /my regexp/,
          new StaleWhileRevalidate({
              plugins: [
                  new BroadcastUpdatePlugin({
                      headersToCheck: ['etag']
                  }),
              ],
          })
      );
  </code></pre>
  
  <p>Thanks for any helping.</p>