Sass won't compile variable if property starts with double dash

I have updated the sass ruby gem to the latest version. Now when compiling .scss files the compiler won't replace the variables by its values if the property name before the variable (the property's value) starts with double dash --:

Example (source.scss):

$header-height: 58px;
$accent: red;

paper-tabs {
    height: $header-height;
    --paper-tabs-selection-bar-color: $accent;
}

Expected output (output.css):

paper-tabs {
    height: 58px;
    --paper-tabs-selection-bar-color: red;
}

Undesired output (output.css):

paper-tabs {
    height: 58px;
    --paper-tabs-selection-bar-color: $accent;
}

Am I doing something wrong? Can I correct it somehow? Thank you.