`/list` endpoint is not listing morph modes

I have a docker image with a single apertium language module.

When `/list` is queries explicitly for analysers and generators, it reports their availability:
```
$ curl 'http://localhost:2738/list?q=analyzers'
{"xyz": "xyz-morph"}
$ curl 'http://localhost:2738/list?q=generators'
{"xyz": "xyz-gener"}
```

Use of the modes also works. However, when I query `/list` without specifying type, it does not report the analysers and generators:

```
$ curl 'http://localhost:2738/list'
{"responseData": [], "responseDetails": null, "responseStatus": 200}
```

According to [the documentation](https://wiki.apertium.org/wiki/Apertium-apy#Usage), `/list` should be returning all mode names, not just language pairs.

I can't figure out if this unexpected (for me) behaviour is a bug, incorrect/out-of-date documentation, or a mis-configuration on my part.