When attempting to filter for the failed tokens after a send using the example in the documentation i keep getting the following

push\server.js:30
      var failed_tokens = gcmdevices.filter((token, i) => response[i].error != null);
                                                                      ^

TypeError: Cannot read property 'error' of undefined

      var failed_tokens = response.results
      .map((res, i) => res.error ? gcmdevices[i] : null)
      .filter(token => token);
      console.log('These tokens are no longer ok:', failed_tokens);