Images from S3 not working with Service Worker

I am in the process of turning one of our websites into a PWA. It has a lot of images from S3 that fail with the following console warning:

The FetchEvent for "[image url]" resulted in a network error response: an object that was not a Response was passed to respondWith().

Here's the SW code:

self.addEventListener('fetch', event => {

  function onFetch (event) {
    // Determine type of asset
    var request = event.request,
        acceptHeader = request.headers.get('Accept'),
        resourceType = 'static';

    if(acceptHeader.indexOf('text/html') !== -1) {
      resourceType = 'content';
    } else if(acceptHeader.indexOf('image') !== -1) {
      resourceType = 'image';
    }

    // Network first for HTML and images
    if(resourceType === 'content' || resourceType === 'image') {
      event.respondWith(fetch(request)
        .then(response => addToCache(request, response)) // read through caching
        .catch(() => fetchFromCache(event))
        .catch(() => offlineResponse(resourceType))
      )
    }
  }

  onFetch(event);

});

function addToCache(request, response) {

  if(response.ok) { // only 200s
    var copy = response.clone(); // Because responses can only be used once
    caches.open(cacheName)
      .then(cache => {
        cache.put(request, copy);
      });

    return response;
  }

}

function fetchFromCache (event) {

  return caches.match(event.request)
    .then(response => {
      if(!response) {
        // A synchronous error that will kick off the catch handler
        throw Error('${event.request.url} not found in cache');
      }
    return response;
  });

}

This only happens with images from S3, all others work as expected. Anyone know what's going on?