Uncaught `Cannot read property 'getHeaders' of undefined` exception
Stacktrace:

```
TypeError: Cannot read property 'getHeaders' of undefined
  File "/home/ubuntu/turnout/turnoutapi/node_modules/koa/lib/response.js", line 49, col 23, in Object.get header [as header]
    return typeof res.getHeaders === 'function'
  File "/home/ubuntu/turnout/turnoutapi/node_modules/koa/lib/response.js", line 203, col 22, in Object.get length [as length]
    const len = this.header['content-length'];
  File "/home/ubuntu/turnout/turnoutapi/node_modules/koa-logger/index.js", line 77, col 33, in logger
    const length = ctx.response.length
  File "async /home/ubuntu/turnout/turnoutapi/src/error.service.js", line 10, col 9, in null.<anonymous>
  File "async /home/ubuntu/turnout/turnoutapi/src/index.js", line 49, col 9, in null.<anonymous>
```

Version: 
```
    "koa": "^2.5.2",
```

Affects:
Doesn't seem to break anything, but gets logged a bunch in my error logging (Sentry)