Error: ENOENT: no such file or directory, open '/...broccoli_merge_trees/assets/assetMap.js'

I'm running into an issue using this addon with Fastboot, whereby I'm getting the following error:

```
Error: ENOENT: no such file or directory, open '/var/folders/5t/y4lxls052bn3ty70r4n7t5b00000gn/T/broccoli-36766MWDqPCH7NGuv/out-1103-broccoli_merge_trees/assets/assetMap.js'
at Object.openSync (fs.js:440:3)
at Object.readFileSync (fs.js:342:35)
at /Users/adam/Projects/tutors/frontend/node_modules/fastboot/src/ember-app.js:189:27
at Array.forEach ()
at EmberApp.loadAppFiles (/Users/adam/Projects/tutors/frontend/node_modules/fastboot/src/ember-app.js:187:21)
at EmberApp.retrieveSandboxedApp (/Users/adam/Projects/tutors/frontend/node_modules/fastboot/src/ember-app.js:235:10)
at new EmberApp (/Users/adam/Projects/tutors/frontend/node_modules/fastboot/src/ember-app.js:61:21)
at FastBoot._buildEmberApp (/Users/adam/Projects/tutors/frontend/node_modules/fastboot/src/index.js:114:17)
at new FastBoot (/Users/adam/Projects/tutors/frontend/node_modules/fastboot/src/index.js:52:10)
at /Users/adam/Projects/tutors/frontend/node_modules/ember-cli-fastboot/index.js:335:29
at Layer.handle [as handle_request] (/Users/adam/Projects/tutors/frontend/node_modules/express/lib/router/layer.js:95:5)
at trim_prefix (/Users/adam/Projects/tutors/frontend/node_modules/express/lib/router/index.js:317:13)
at /Users/adam/Projects/tutors/frontend/node_modules/express/lib/router/index.js:284:7
at Function.process_params (/Users/adam/Projects/tutors/frontend/node_modules/express/lib/router/index.js:335:12)
at next (/Users/adam/Projects/tutors/frontend/node_modules/express/lib/router/index.js:275:10)
at /Users/adam/Projects/tutors/frontend/node_modules/ember-cli/lib/tasks/server/middleware/broccoli-watcher/index.js:54:11
at watcher.then.errorHandler.buildError (/Users/adam/Projects/tutors/frontend/node_modules/broccoli-middleware/lib/watcher-middleware.js:35:7)
at processTicksAndRejections (internal/process/task_queues.js:93:5)
```

In `environment.js`, we have:
```
ifa: {
enabled: false,
inline: true
}
```

And in `ember-cli-build.js`, we have:
```
fingerprint: {
prepend: 'https://cdn.tutorspot.co.uk/',
extensions: [...],
exclude: [...],
generateAssetMap: true,
fingerprintAssetMap: true
},
```

This error is only showing up in Fastboot. Everything works fine running locally with Fastboot disabled.

Any guidance would be greatly appreciated!