Unable to properly invoke markdown-pdf

Hello,
just updated and tried to use it again with very simple invocation:

`markdown-pdf doc.md`

I expected a `doc.pdf` to be created. Instead I got the following error message:

```
internal/validators.js:118
throw new ERR_INVALID_ARG_TYPE(name, 'string', value);
^

TypeError [ERR_INVALID_ARG_TYPE]: The "file" argument must be of type string. Received null
at validateString (internal/validators.js:118:11)
at normalizeSpawnArguments (child_process.js:406:3)
at spawn (child_process.js:546:13)
at Object.execFile (child_process.js:232:17)
at WriteStream. (/usr/local/lib/node_modules/markdown-pdf/index.js:117:22)
at WriteStream.emit (events.js:333:22)
at finishMaybe (_stream_writable.js:697:14)
at _stream_writable.js:673:7
at WriteStream._final (internal/fs/streams.js:377:3)
at callFinal (_stream_writable.js:666:10) {
code: 'ERR_INVALID_ARG_TYPE'
}
```

Aside it created an empty `doc.pdf` file, also indicating that it's not a permission issue on my system.
I am using macOS Catalina.

Cheers,
Florian