I updated an old Next.js app and when I ran npm run dev I had this error:

error - ./node_modules/next/dist/build/webpack/loaders/css-loader/src/index.js??ruleSet[1].rules[2].oneOf[8].use[1]!./node_modules/next/dist/build/webpack/loaders/postcss-loader/src/index.js??ruleSet[1].rules[2].oneOf[8].use[2]!./styles/globals.css
Error: Your custom PostCSS configuration must export a `plugins` key.

I added a postcss.config.json file with this content:

{
  "plugins": []
}

and the app started working again.

Lessons in this unit:

0:	Introduction
1:	Blank page after router.push() in Next.js?
2:	How to fix the error `PrismaClient is unable to be run in the browser` in Next.js
3:	Next.js, blank page after calling `res.redirect()`
4:	Next.js, how to fix the error `Constructor requires 'new' operator`
5:	Next.js, fix the `module not found` error
6:	How to fix the `can't resolve module` error in Next.js
7:	How to fix error serializing Date object JSON in Next.js
8:	How to fix the `unable to resolve dependency tree` PostCSS and Tailwind issue in Next.js
9:	Fix “Module not found: Can't resolve encoding” in Next.js
10:	▶︎ How to fix the error "Your custom PostCSS configuration must export a `plugins` key."
11:	Next.js, what to do when the state of a component is not refreshed when navigating
12:	Revalidation and ISR gotcha on Vercel
13:	How to fix the `Already 10 Prisma Clients are actively running` error