Debugging the Blind: How to Fix Next.js cssnano-simple Build Crashes

The Blind Spot of Next.js Production Builds

You run next build. You expect a smooth CI/CD deployment. Instead, your console spits out a single, cryptic line:
HookWebpackError: Cannot read properties of undefined in cssnano-simple

No file reference. No line number. No component stack. You are officially debugging "blind."

Why cssnano-simple Crashes

Next.js uses a specialized, high-performance minifier called cssnano-simple to compress your stylesheets for production. This minifier is blazing fast because it assumes your CSS is perfectly formed. However, when it encounters an invalid or unexpected Abstract Syntax Tree (AST) node—often generated by complex Tailwind dynamic utilities or malformed CSS-in-JS—it hits a null reference and panics.

Common Culprits

• Empty @apply directives: An @apply with no arguments or only comments can result in an empty rule that triggers the crash.
• Malformed CSS Variables: Native CSS variables that are opened but never closed properly.
• Tailwind Plugin Conflicts: Third-party plugins that output non-standard CSS properties.

The Diagnostic Workflow: A Case Study

In a recent real-world case flagged by our automated telemetry, a developer was stuck for 4 hours on this exact issue. The fix wasn't in the code they had just written, but in a legacy globals.css file where a stray & was used outside of a nesting context.

Phase 1: Binary Isolation

Since the error gives no path, you must find it yourself. Comment out half of your CSS imports in _app.tsx or layout.tsx. Run build. If it passes, the bug is in the half you commented out. Repeat this "binary search" until you find the offending file.

Phase 2: JIT Inspection

Check your tailwind.config.js. If you are using advanced theme extensions, verify that no function returns undefined. Tailwind’s JIT engine will happily put that literally into your CSS, but cssnano-simple will choke on it.

Conclusion

Cryptic build errors are a rite of passage for Next.js developers. By understanding that cssnano-simple is a "fast but fragile" parser, you can stop looking for logic bugs and start looking for syntax anomalies.

Need to validate your CSS structure? Use our CSS Minifier & Validator to catch AST errors before they hit your production build pipeline.