I'm having a webpacker issue with my app on heroku.

The log:

```
ActionView::Template::Error (Webpacker can't find application.js in /app/public/packs/manifest.json. Possible causes:
2018-07-04T18:47:08.192729+00:00 app[web.1]: 1. You want to set webpacker.yml value of compile to true for your environment
2018-07-04T18:47:08.192730+00:00 app[web.1]:    unless you are using the `webpack -w` or the webpack-dev-server.
2018-07-04T18:47:08.192733+00:00 app[web.1]: 2. webpack has not yet re-run to reflect updates.
2018-07-04T18:47:08.192734+00:00 app[web.1]: 3. You have misconfigured Webpacker's config/webpacker.yml file.
2018-07-04T18:47:08.192735+00:00 app[web.1]: 4. Your webpack configuration is not creating a manifest.
2018-07-04T18:47:08.192737+00:00 app[web.1]: Your manifest contains:
2018-07-04T18:47:08.192738+00:00 app[web.1]: {
2018-07-04T18:47:08.192740+00:00 app[web.1]: }
2018-07-04T18:47:08.192741+00:00 app[web.1]: ):

```

I have a `manifest.json` file which does contain `application.js`:

```
{
  "application.css": "/packs/application-c922057d6b9db34084d6e52c6f32baa9.css",
  "application.css.map": "/packs/application-c922057d6b9db34084d6e52c6f32baa9.css.map",
  "application.js": "/packs/application-ab33c1479403810d7775.js",
  "application.js.map": "/packs/application-ab33c1479403810d7775.js.map"
}

```

It's interesting the final line in the log:

```
2018-07-04T18:47:08.192737+00:00 app[web.1]: Your manifest contains:
2018-07-04T18:47:08.192738+00:00 app[web.1]: {
2018-07-04T18:47:08.192740+00:00 app[web.1]: }

```

Is the formatting in the json wrong somehow? 

I've tried running `bundle exec rails webpacker:compile` which was fine, and I've also updated the binstub `bundle exec rails webpacker:binstubs`, neither seem to have made a difference after pushing to Heroku.

What else can I try?