Deploy warning calling external JS function

So, in my serverless.yml file I have this:

  my_attr: ${file(./serverless/get-custom-value.js):my_attr}

And in that file (./serverless/get-custom-value.js) is this JavaScript code:

module.exports.my_attr = async function(slsArg) {
  const stage =
  console.debug(`### stage: "${stage}".`)
  return stage

When doing a package or deploy (which is successful), I see this warning:

Serverless: Deprecation warning: Variables resolver reports following resolution errors:
              - Cannot resolve variable at "custom.my_attr": Cannot resolve "my_attr" out of "get-custom-value.js": Resolved a JS function not confirmed to work with a new parser, falling back to old resolver

Yet, despite the warning it works exactly as expected…