Compile fails when building both renderer and main user `compile` but succeeds when run separately

If I run `electron-webpack` my build fails complaining about `renderer/index.js` not having a default export.

```
Child
--
  | Hash: f01ed1cc57d9d84193b0
  | Time: 1283ms
  | Built at: 2018-06-27 08:50:59
  | 2 assets
  | Entrypoint renderer = renderer.js renderer.js.map
  | [0] ./src/renderer/index.js 2.57 KiB {0} [built] [failed] [1 error]
  | [1] multi ./src/renderer/index.js 28 bytes {0} [built]
  |  
  | ERROR in ./src/renderer/index.js
  | Module build failed: Error: Must export a default export when using ES6 modules.
  | at createDescriptor (/usr/local/var/buildkite-agent/builds/Britts-MBP.illtemper.org-1/strongdm/development-client/clientui/node_modules/@babel/core/lib/config/config-descriptors.js:172:13)
```

However if I run `electron-webpack main` then `electron-webpack renderer` it builds fine. I tried adding a default export just to see if that was the issue but it didn't fix anything. So, my suspicion is that the message is covering up for another error.