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HookWebpackError: Cannot read properties of undefined in cssnano-simpleThis is a notorious Next.js build-time error that occurs during the CSS minification phase. Since Next.js uses 'cssnano-simple' for production optimization, any malformed CSS structure (often generated by Tailwind dynamic classes or custom PostCSS plugins) can cause the minifier's AST parser to crash with an 'undefined' reference. The error is particularly frustrating because it rarely provides a file path.
Temporarily comment out your global CSS imports one by one to see which file triggers the crash during 'next build'.
If you recently added a plugin or a custom theme extension, verify that it isn't outputting invalid CSS values (like undefined or null).
Ensure you are using the default Next.js PostCSS configuration unless absolutely necessary.
module.exports = {
plugins: {
'tailwindcss': {},
'autoprefixer': {},
},
}Ensure no CSS file contains an @apply directive with no arguments or only comments.
The cssnano-simple minifier is designed for speed, not robustness. When it encounters a CSS tree that doesn't follow standard specifications—which can happen when CSS-in-JS or Tailwind JIT processes 'cheat' the parser—it fails to find the expected nodes in the Abstract Syntax Tree (AST).
Because the error happens inside the Webpack Minification Hook, the original source-map context is often already lost. The minifier is looking at a massive, concatenated string of all your CSS. To debug, you must work backward from your globals.css and any third-party UI libraries (like Shadcn or Radix) you've recently updated.