postcss-scss: cannot parse @mixin syntax ?

@mixin border-radius($radius) { ^ -webkit-border-radius: $radius;

ERROR in ./~/css-loader?sourceMap&modules&localIdentName=[name]_[local]_[hash:base64:3]!./~/postcss-loader?parser=postcss-scss!./src/components/SearchWidget/SearchWidget.scss
Module build failed: TypeError: Cannot read property 'toString' of undefined
    at new Input