Build fails with "Element type is invalid" error when using serverComponentsExternalPackages: ["ably"] in next.config.js

Details:
- Next.js 14.2.29 App Router
- Works fine in dev (npm run dev)
- Fails during build (npm run build) with prerender errors
- Even with ALL Ably code commented out, just having the config breaks the build

Config:

// next.config.js
experimental: {
  serverComponentsExternalPackages: ["ably"]
}

Error:
Error occurred prerendering pages.
Element type is invalid: expected a string (for built-in components)
or a class/function (for composite components) but got: undefined.