TypeError: Cannot read properties of undefined (reading 'entryCSSFiles') Bug handling NotFound pages when using pageExtensions config

[heymath]

Link to the code that reproduces this issue

https://github.com/heymath/nextjs-bug-not-found-demo

To Reproduce

1. start the app of the demo repo
2. go on http://localhost:3000/test/?notFound=1
3. you get a runtime error "TypeError: Cannot read properties of undefined (reading 'entryCSSFiles')"

Current vs. Expected behavior

Right now you get an Runtime Error "TypeError: Cannot read properties of undefined (reading 'entryCSSFiles')" instead of seeing a NotFound page.

Four our production app it results in a blank page "Internal server error"

Provide environment information

Operating System:
  Platform: linux
  Arch: x64
  Version: #31~22.04.1-Ubuntu SMP PREEMPT_DYNAMIC Wed Aug 16 13:45:26 UTC 2
  Available memory (MB): 15705
  Available CPU cores: 16
Binaries:
  Node: 22.12.0
  npm: 10.9.0
  Yarn: N/A
  pnpm: N/A
Relevant Packages:
  next: 15.2.0 // Latest available version is detected (15.2.0).
  eslint-config-next: 15.2.0
  react: 19.0.0
  react-dom: 19.0.0
  typescript: 5.7.3
Next.js Config:
  output: N/A

Which area(s) are affected? (Select all that apply)

Error Handling

Which stage(s) are affected? (Select all that apply)

next dev (local), next start (local)

Additional context

Demo repository with Next.js v15.2, we have the same bug in production with v15.1.

I was able to reproduce our bug when combining pageExtensions + trailingSlash in next.config.ts so I limited the Demo repository to this setup and the use of PagesRouter + AppRouter because we are using both in our production App.

Note

Edit: I realized that the bug is only cause by the pageExtensions config actually.

Error screenshot instead of 404 page

[heymath]

I just tested with Next v14.2.24 (and React / ReactDOM v18)

And it has the same buggy behavior, it has just a different error message, see:

[heymath]

We stopped using the page extension to unlock the problem...
We move all our code in src/pages/ out of the directory with a big PR

[tebb]

In the unlikely event that this helps ... I was getting the error:

TypeError: Cannot read properties of undefined (reading 'entryCSSFiles')

when using the app router.

The issue appeared to be using the word 'index' in the path.

Changing /index/[id] to /blog/[id] allowed the build to complete.

There was a further issue of css not being applied which was resolved by deleting .next directory and re-running the build.