Unable to generate C# types for GitHub's webhook schema

I'm attempting to generate C# types for GitHub's webhook schema. There is no official schema, but they have been generated [here](https://github.com/octokit/webhooks/tree/master/payload-schemas), and I am specifically attempting to [use this file](https://unpkg.com/@octokit/webhooks-schemas@4.2.1/schema.json).

When running the following command:

```sh
quicktype --lang cs --out webhooks.cs --src-lang schema --debug all schema.json
```

I get the error:

```sh
Error: Trying to make an empty union - do you have an impossible type in your schema?.
```

I've uploaded the [full debug output here](https://gist.github.com/JamieMagee/8a3a27c758bdbde77a1c86a368d368e8#file-schema-log-txt), and my quicktype version info is:

```sh
$ quicktype --version
quicktype version 15.0.260
```

I've also attempted to convert from [the TypeScript types](https://github.com/octokit/webhooks/tree/master/payload-types) that are generated from the JSON schema, using the command:

```sh
quicktype --lang cs --out webhooks.cs --src-lang typescript --debug all schema.d.ts
```

quicktype exits with no error ([debug log here](https://gist.github.com/JamieMagee/8a3a27c758bdbde77a1c86a368d368e8#file-typescript-log-txt)), but no POCOs are generated ([see the complete output here](https://gist.github.com/JamieMagee/8a3a27c758bdbde77a1c86a368d368e8#file-typescript-webhooks-cs))