Unknown LESS variable when injected from a mixin, inside Visual Studio and with Cassette

Question

I just purchased an HTML template and it has this LESS code:

.shop-builder(@container, @columns, @margin) {
    @width: floor( ( @container - ( @margin * ( @columns - 1 ) ) ) / @columns );
    @height: floor( ( @width * 4 ) / 3 );
}

.shop-item-width(@container; @columns; @margin) {
    .shop-builder(@container, @columns, @margin);
    width: @width;
}

Visual Studio 2015 underlines an error in "width: @width" saying "unknown variable". When the asp.net MVC app (with cassette and its LESS plugin using the latest DotlessClientOnly nuget package) runs, I get the same error. I checked the LESS syntax and it seems it is correct to use a variable from a mixin...

Any idea?