Could not resolve variable $c within #{$c}__label{ ... }

Hi all,

I'm switching over from SCSS in BEM-notation to PostCSS and run into the following issue:

I declare the main selector as a $c: &; so I can use it throughout the rest of my .pcss file, but when I try to access it within something else inside that selector like #{$c}__label{ ... } I get the message:

``` Internal server error: postcss-advanced-variables: /Users/xxx/Sites/xxx/resources/styles/07-components/_primary-nav.pcss:13:16: Could not resolve the variable "$c" within "#{$c}__label"```

Here's a screenshot of what I'm trying to do: [screenshot](https://share.getcloudapp.com/Koudo1Wn)

Don't know if anyone has any tips to achieve something like this? Thanks!