TypeError: Cannot read property 'toLowerCase' of undefined

[davidbrooksio]

I'm trying to build my project using webpack 4.44.2 and keep getting this error:

ERROR in ./src/index.css (./node_modules/css-loader/dist/cjs.js??ref--9-1!./node_modules/postcss-loader/dist/cjs.js!./src/index.css)
Module build failed (from ./node_modules/postcss-loader/dist/cjs.js):
TypeError: Cannot read property 'toLowerCase' of undefined
    at C:\<path to folder>\node_modules\postcss-advanced-variables\index.js:588:31
 @ ./src/index.css 2:26-142
 @ ./src/index.js

My dependencies are:

"devDependencies": {
    "@babel/core": "^7.11.6",
    "@babel/plugin-syntax-dynamic-import": "^7.8.3",
    "@babel/polyfill": "^7.11.5",
    "@babel/preset-env": "^7.11.5",
    "@webcomponents/webcomponentsjs": "^2.4.4",
    "babel-loader": "^8.1.0",
    "clean-webpack-plugin": "^3.0.0",
    "copy-webpack-plugin": "^6.1.1",
    "css-loader": "^4.3.0",
    "cssnano": "^4.1.10",
    "esdoc": "^1.1.0",
    "esdoc-standard-plugin": "^1.0.0",
    "eslint": "^7.9.0",
    "file-loader": "^6.1.0",
    "html-loader": "^1.3.1",
    "html-webpack-plugin": "^4.5.0",
    "html2hscript": "^2.0.1",
    "hyperscript": "^2.0.2",
    "mini-css-extract-plugin": "^0.11.2",
    "normalize.css": "^8.0.1",
    "postcss": "^8.0.7",
    "postcss-color-mod-function": "^3.0.3",
    "postcss-each": "^0.10.0",
    "postcss-easing-gradients": "^3.0.1",
    "postcss-import": "^12.0.1",
    "postcss-inline-svg": "^4.1.0",
    "postcss-loader": "^4.0.2",
    "postcss-percentage": "0.0.0",
    "postcss-reporter": "^7.0.0",
    "precss": "^4.0.0",
    "style-loader": "^1.2.1",
    "terser-webpack-plugin": "^4.2.2",
    "webpack": "^4.44.2",
    "webpack-cli": "^3.3.12",
    "webpack-dev-server": "^3.11.0",
    "webpack-manifest-plugin": "^2.2.0",
    "webpack-merge": "^5.1.4"
  },

My postcss.config.js file is:

module.exports = {
  plugins: [
    require('postcss-import'),
    require('postcss-each'),
    require('postcss-color-mod-function'),
    require('precss'),
    require('postcss-percentage')({
      precision: 3,
      trimTrailingZero: true,
      floor: true
    }),
    require('postcss-easing-gradients'),
    require('postcss-inline-svg')({
      paths: ['src/img/', 'node_modules/feather-icons/dist/icons/'],
      removeFill: true,
      encode: false
    }),
    require('cssnano')({
      preset: [
        'default',
        {
          discardComments: {
            removeAll: true
          },
          colorMin: false,
          mergeLonghand: false,
          mergeRules: false
        }
      ]
    })
  ]
}

I'm not used to filing bug reports so sorry if this one is not correct.

[romainmenke]

I can no longer reproduce this so I am assuming it has already been fixed.