App Router app with a route at index/page.tsx | Next.js | Page 1

I've got a Next 15 app and would like to have a route at /index (separate to the root /). My folder structure is all standard but the page.tsx in the app/index/ folder hits an error on build:

Error occurred prerendering page "/index".
[Error [InvariantError]: Invariant: Expected clientReferenceManifest to be defined. This is a bug in Next.js.]
Export encountered an error on /index/page: /index, exiting the build.

The /index route works in dev mode. Is there any extra config needed for a successful build, or is a route named /index not possible? Thanks

#App Router app with a route at index/page.tsx