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

support
2

1st. check against

Is your css really a template?

2nd. for a single CSS is looks like

{{ $mccs := resources.Get "css/main.css" | minify }}{{ $secureMCCS := $mccs | resources.Fingerprint site.Params.digest }}
    <link type=text/css rel=stylesheet href={{ $secureMCCS.RelPermalink }} integrity={{ $secureMCCS.Data.Integrity }} media="screen, print" />

3rd. for appending files search the forum

found first this