"WebpackOptionsValidationError: Invalid configuration object." when building a local package

[typeofweb]

WebpackOptionsValidationError: Invalid configuration object. Webpack has been initialised using a configuration object that does not match the API schema.
  Error: WebpackOptionsValidationError: Invalid configuration object. Webpack has been initialised using a configuration object that does not match the API schema.
   - configuration.module.rules[5].loader should be one of these:
     non-empty string | non-empty string | function | object { ident?, loader?, options?, query? } | function | [non-empty string | function | object { ident?, loader?, options?, query? }]

Full output is available here: https://github.com/typeofweb/schema/runs/1710304249?check_suite_focus=true

Did I do anything wrong? This is how I use the package:

https://github.com/typeofweb/typeofweb-bundlephobia-pr-stats-action/blob/a04a9c61d4d3f8a528dddbba1e54afa3de091834/src/build.ts#L31-L41

And this is the package I'm trying to measure:
https://github.com/typeofweb/schema