(plugin postcss) TypeError: Cannot read property 'syntax' of undefined

```
export default {
entry: './index.js',
plugins: [
stylelint({
include: ["./**/*.{css}"],
}),
postcss({
extract: false,
inject: true,
plugins: [cssUrl({url: "inline"})],
minimize: isProduction,
modules: true,
sourceMap: isProduction ? "inline" : ""
}),
]
},

```
When postcss minimize option is set to `true` the following fatal error occurs:

[!] (plugin postcss) TypeError: Cannot read property 'syntax' of undefined
components/SliderVertical/Slider.css
TypeError: Cannot read property 'syntax' of undefined
at Declaration.toString (/home/me/test/node_modules/postcss-syntax/patch-postcss.js:38:67)
at String ()