How to debug local transformer not running

[wenkokke]

I have some local transformer plugin, which compiles Markdown to HTML using Pandoc:

// file: plugins/parcel-transformer-pandoc.mjs
import { Transformer } from '@parcel/plugin';

export default new Transformer({
  async transform({ asset, config }) {
    ...
  }
});

And the following .parcelrc file:

{
  "extends": [
    "@parcel/config-default"
  ],
  "transformers": {
    "*.md": [
      "./plugins/parcel-transformer-pandoc.mjs"
    ]
  }
}

However, when I run parcel build src/index.md, the plugin doesn't run and Parcel tries to run the standard MarkdownAsset pipeline, which results in:

[12:17:05 PM]: Building...
[12:17:05 PM]: Building index.md...
[12:17:05 PM]: 🚨  src/index.md: marked is not a function

I can't debug what's happening in my plugin because, as far as I can tell, it isn't running at all, but Parcel doesn't throw any errors at all.

How do I debug this?