Invalid PostCSS Configuration

Why This Error Occurred

PostCSS configuration was provided in an unsupported shape.

Possible Ways to Fix It

PostCSS configuration must be defined in the following shape:

module.exports = {
  plugins: [
    // A plugin that does not require configuration:
    'simple-plugin-example',

    // A plugin which needs a configuration object:
    [
      'plugin-with-configuration',
      {
        optionA: '...',
      },
    ],

    // A plugin that is toggled on or off based on environment:
    [
      'plugin-toggled',
      process.env.NODE_ENV === 'production'
        ? {
            optionA: '...',
          }
        : false,
    ],

    // Boolean expressions are also valid.
    // `true` enables the plugin, `false` disables the plugin:
    ['plugin-toggled-2', true /* a === b, etc */],
  ],
}

You can read more about configuring PostCSS in Next.js here.

Common Errors

Before: plugin is require()'d

const pluginA = require('postcss-plugin-a')
module.exports = {
  plugins: [require('postcss-plugin'), pluginA],
}

After

module.exports = {
  plugins: ['postcss-plugin', 'postcss-plugin-a'],
}

Before: plugin is instantiated with configuration

module.exports = {
  plugins: [
    require('postcss-plugin')({
      optionA: '...',
    }),
  ],
}

After

module.exports = {
  plugins: [
    // Pay attention to this nested array. The first index is the plugin name,
    // the second index is the configuration.
    [
      'postcss-plugin',
      {
        optionA: '...',
      },
    ],
  ],
}

Before: plugin is missing configuration

module.exports = {
  plugins: [
    [
      'postcss-plugin-1',
      {
        optionA: '...',
      },
    ],
    // This single-entry array is detected as misconfigured because it's
    // missing the second element. To fix, unwrap the value.
    ['postcss-plugin-2'],
  ],
}

After

module.exports = {
  plugins: [
    [
      'postcss-plugin-1',
      {
        optionA: '...',
      },
    ],
    // Only string:
    'postcss-plugin-2',
  ],
}