Turbopack fails to evaluate variable-assigned default exports in postcss.config.mjs

Link to the code that reproduces this issue

https://github.com/amoh909/nextjs-turbopack-postcss-bug

To Reproduce

1. Create a postcss.config.mjs file at the root of a Next.js project with the following structure:

   const config = {
     plugins: {
       "@tailwindcss/postcss": {},
     },
   };
   export default config;
   

2. Start the local development server (which executes Turbopack).
3. The following trace will be printed upon loading globals.css:

   Error: Error evaluating Node.js code
   Error: PostCSS config is undefined (make sure to export an function or object from config file)
       [at Module.init (turbopack:///[turbopack-node]/transforms/postcss.ts?config=[project]/postcss.config.mjs:23:11)]
   
   

Current vs. Expected behavior

Current Behavior:
The Next.js development server fails to compile CSS files (globals.css and page.module.css), repeatedly printing a low-level compilation error to the terminal terminal: Error: Error evaluating Node.js code followed by Error: PostCSS config is undefined. The dev server remains alive, but styles do not process because Turbopack fails to read the config object when it is assigned to an intermediate variable prior to being exported.

Expected Behavior:
Turbopack's configuration evaluator should successfully parse and resolve the variable reference for the default export (const config = ...; export default config;), mirroring standard Node.js module resolution, allowing the CSS assets to compile cleanly without throwing terminal errors.

Provide environment information

Operating System:
  Platform: win32
  Arch: x64
  Version: Windows 11 Pro
  Available memory (MB): 16109
  Available CPU cores: 16
Binaries:
  Node: 24.12.0
  npm: 11.6.2
  Yarn: N/A
  pnpm: 11.14.0
Relevant Packages:
  next: 16.3.0 // Latest available version is detected (16.3.0).
  eslint-config-next: N/A
  react: 19.2.8
  react-dom: 19.2.8
  typescript: 5.9.3
Next.js Config:
  output: N/A

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

CSS

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

next dev (local)

Additional context

The configuration file itself is completely valid JavaScript syntax. Running the module directly through the Node.js runtime successfully resolves and extracts the object:

node -e "import('./postcss.config.mjs').then(console.log)"

Output: [Module: null prototype] { default: { plugins: { '@tailwindcss/postcss': {} } } }

Changing the file structure to use a direct inline object literal expression (export default { plugins: { ... } };) completely bypasses the terminal error loop without needing to revert Next.js versions.

Start by running the linked reproduction with next dev and inspecting the failing turbopack:///[turbopack-node]/transforms/postcss.ts entry point. Compare handling of the direct inline export with the variable-assigned export in postcss.config.mjs; done means globals.css and page.module.css compile without the PostCSS config error.