Custom CSS throws: Type <nil> not supported in Resource transformations

My css isn’t a template, it’s just a full css.
The piece of the previous was copied, as last resort, from another post.
You solution, mentioned as point 2, is what I’ve tried already with an error.
I’ve figured it out now:

  1. The initial css I was using was already minified, apparently the ‘double minification’ doesn’t work very well.
  2. The CSS was working good when added to a static HTML page. Hugo raise an error processing it, there’s a semicolon unneeded at the end.

Thanks for the hint.