CSS error breaks optimization

Hello. This code can't be minified:
```css
.marquee p:nth-child {
    color:red;
}
```
The error text:
```
/usr/lib/node_modules/csso-cli/node_modules/csso/cjs/restructure/prepare/specificity.cjs:80
                        const arg = node.children.first;
```
Online service https://css.github.io/csso/csso.html shows error too.