[Next.js 15] CSS Minification fails with "Unclosed block" error during production build

[apiwatja]

Link to the code that reproduces this issue

https://github.com/apiwatja/20250209-css-minification-issue.git

To Reproduce

Reproduction Steps

Create a Next.js 15 project with Tailwind CSS
Install vaul package
Run production build
Build fails during CSS minification

Current vs. Expected behavior

Issue

Production builds fail when CSS minification is enabled in Next.js 15. The error points to an "Unclosed block" issue during the minification process. While direct CSS minification through cssnano works fine, Next.js's built-in minification process fails.

Error Description

During production build (next build), the process fails with a CSS minification error. The error occurs specifically when processing CSS from the vaul package's style.css.

GitHub Action Error:

#18 1.308    ▲ Next.js 15.1.6
#18 1.308    - Environments: .env.local
#18 1.308 
#18 1.381    Creating an optimized production build ...
#18 155.9 Failed to compile.
#18 155.9 
#18 155.9 HookWebpackError: /usr/src/app/static/css/9d611f82b4eb5c2d.css:1:100501: Unclosed block
#18 155.9     at makeWebpackError (/usr/src/app/node_modules/next/dist/compiled/webpack/bundle5.js:28:314608)
#18 155.9     at /usr/src/app/node_modules/next/dist/compiled/webpack/bundle5.js:28:106402
#18 155.9     at eval (eval at create (/usr/src/app/node_modules/next/dist/compiled/webpack/bundle5.js:13:9218), <anonymous>:44:1)
#18 155.9 -- inner error --
#18 155.9 CssSyntaxError: /usr/src/app/static/css/9d611f82b4eb5c2d.css:1:100501: Unclosed block
#18 155.9     at Input.error (/usr/src/app/node_modules/next/node_modules/postcss/lib/input.js:106:16)
#18 155.9     at ScssParser.unclosedBlock (/usr/src/app/node_modules/next/node_modules/postcss/lib/parser.js:574:22)
#18 155.9     at ScssParser.endFile (/usr/src/app/node_modules/next/node_modules/postcss/lib/parser.js:336:35)
#18 155.9     at ScssParser.parse (/usr/src/app/node_modules/next/node_modules/postcss/lib/parser.js:475:10)
#18 155.9     at scssParse (/usr/src/app/node_modules/next/dist/compiled/postcss-scss/scss-syntax.js:1:322)
#18 155.9     at new LazyResult (/usr/src/app/node_modules/next/node_modules/postcss/lib/lazy-result.js:133:16)
#18 155.9     at Processor.process (/usr/src/app/node_modules/next/node_modules/postcss/lib/processor.js:53:14)
#18 155.9     at CssMinimizerPlugin.optimizeAsset (/usr/src/app/node_modules/next/dist/build/webpack/plugins/css-minimizer-plugin.js:48:12)
#18 155.9     at /usr/src/app/node_modules/next/dist/build/webpack/plugins/css-minimizer-plugin.js:79:55
#18 155.9     at async Span.traceAsyncFn (/usr/src/app/node_modules/next/dist/trace/trace.js:153:20)
#18 155.9 caused by plugins in Compilation.hooks.processAssets
#18 155.9 CssSyntaxError: /usr/src/app/static/css/9d611f82b4eb5c2d.css:1:100501: Unclosed block
#18 155.9     at Input.error (/usr/src/app/node_modules/next/node_modules/postcss/lib/input.js:106:16)
#18 155.9     at ScssParser.unclosedBlock (/usr/src/app/node_modules/next/node_modules/postcss/lib/parser.js:574:22)
#18 155.9     at ScssParser.endFile (/usr/src/app/node_modules/next/node_modules/postcss/lib/parser.js:336:35)
#18 155.9     at ScssParser.parse (/usr/src/app/node_modules/next/node_modules/postcss/lib/parser.js:475:10)
#18 155.9     at scssParse (/usr/src/app/node_modules/next/dist/compiled/postcss-scss/scss-syntax.js:1:322)
#18 155.9     at new LazyResult (/usr/src/app/node_modules/next/node_modules/postcss/lib/lazy-result.js:133:16)
#18 155.9     at Processor.process (/usr/src/app/node_modules/next/node_modules/postcss/lib/processor.js:53:14)
#18 155.9     at CssMinimizerPlugin.optimizeAsset (/usr/src/app/node_modules/next/dist/build/webpack/plugins/css-minimizer-plugin.js:48:12)
#18 155.9     at /usr/src/app/node_modules/next/dist/build/webpack/plugins/css-minimizer-plugin.js:79:55
#18 155.9     at async Span.traceAsyncFn (/usr/src/app/node_modules/next/dist/trace/trace.js:153:20)
#18 155.9 
#18 155.9 
#18 155.9 > Build failed because of webpack errors
#18 156.3  ELIFECYCLE  Command failed with exit code 1.
#18 ERROR: process "/bin/sh -c pnpm build" did not complete successfully: exit code: 1

Provide environment information

### Environment
Next.js version: 15.1.6
Node.js version: 22.9.0
Operating System: Linux (Docker container)
Package Manager: pnpm 10.2.1

### Dependencies Related to the Issue
tailwindcss: 4.0.2
postcss: 8.4.35
cssnano: 7.0.6
vaul: 1.1.2 (package containing problematic CSS)

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

Webpack

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

Other (Deployed)

Additional context

Additional Context

Direct minification of the CSS files using cssnano works fine
The issue appears to be specific to Next.js 15's built-in minification process
Multiple attempts to configure postcss and cssnano options did not resolve the issue
The error is reproducible in both local and CI/CD environments

Current Workaround

The issue can be worked around by disabling CSS minification in next.config.mjs:

if (!dev) {
  if (!config.optimization) {
    config.optimization = {};
  }
  config.optimization.minimize = false;
  config.optimization.minimizer = [];
}

[connershoop]

I encountered the same CSS minification error in Next.js 14.2.23.
The workaround (config.optimization.minimize = false) also resolved the issue for me, but isn't a valid workaround to deploy with as it introduces production vulnerabilities

My dependencies:

• @tailwindcss/postcss@4.0.6
• @types/node@20.17.17
• @types/react@18.3.18
• @types/react-dom@18.3.5
• autoprefixer@10.4.20
• eslint@8.57.1
• eslint-config-next@15.0.3
• next@14.2.23
• postcss@8.5.1
• react@18.3.1
• react-dom@18.3.1
• tailwindcss@4.0.6
• typescript@5.7.3

Also running on Ubuntu 20.04.6 LTS

[github-actions]

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Useful Resources

• How to Contribute to Open Source (Next.js)
• How to create a Minimal, Complete, and Verifiable example
• Reporting a Next.js bug
• Next.js Triaging issues

[allreddv]

@apiwatja Did you get this resolved? I am getting the same thing all of a sudden and track it done to fix it. It also seems like this came out of nowhere because any code changes I have made recently are not connected to anything css related and I have not installed any new packages or anything.

[jgx-bpowley]

I am getting this same issue.

This only happens when using output: 'standalone' and only happens in Docker. If I run npm run build it works fine, but in Docker is fails.

I have tried running a Docker build from scratch with --no-cache.

I am using tailwind 4.

[danielkoller]

I am having the exact same problem.

• Next 15.1.7
• Tailwind 4.0.6

Maybe we should open a Tailwind-issue also?

[jgx-bpowley]

I have fixed this!

It seems to be something to do with the build process not using the postcss config.

My postcss.config.mjs was not being included in my Docker build. I updated my Dockerfile to include -

COPY postcss.config.mjs ./ in the deps step and it now builds correctly