Error when using file:// uri for stylesdir

I am using Windows 7 and ruby 2.2.

If I use this command, I get no error message, but firefox won't read the css file:

 >asciidoctor -a linkcss -a stylesheet=asciidoctorx.css -a stylesdir=c:\css\ spl.txt

However, if I put the proper uri in for the stylesdir, like this:

 >asciidoctor -a linkcss -a stylesheet=asciidoctorx.css -a stylesdir=file://c:\css\ spl.txt

I get the following error message:

Invalid argument @ dir_s_mkdir - C:/Users/blablabla/Documents/My projects/safety/xxxdir/file:
  Use --trace for backtrace

The conversion works, despite the error message.

Here's the trace:

C:/Ruby22/lib/ruby/gems/2.2.0/gems/asciidoctor-1.5.2/lib/asciidoctor/helpers.rb:147:in `mkdir': Invalid argument @ dir_s_mkdir - C:/Users/blablabla/Documents/My projects/safety/xxxdir/file: (Errno::EINVAL)
        from C:/Ruby22/lib/ruby/gems/2.2.0/gems/asciidoctor-1.5.2/lib/asciidoctor/helpers.rb:147:in `mkdir_p'
        from C:/Ruby22/lib/ruby/gems/2.2.0/gems/asciidoctor-1.5.2/lib/asciidoctor/helpers.rb:145:in `mkdir_p'
        from C:/Ruby22/lib/ruby/gems/2.2.0/gems/asciidoctor-1.5.2/lib/asciidoctor/helpers.rb:145:in `mkdir_p'
        from C:/Ruby22/lib/ruby/gems/2.2.0/gems/asciidoctor-1.5.2/lib/asciidoctor.rb:1516:in `convert'
        from C:/Ruby22/lib/ruby/gems/2.2.0/gems/asciidoctor-1.5.2/lib/asciidoctor/cli/invoker.rb:94:in `block in invoke!'
        from C:/Ruby22/lib/ruby/gems/2.2.0/gems/asciidoctor-1.5.2/lib/asciidoctor/cli/invoker.rb:86:in `each'
        from C:/Ruby22/lib/ruby/gems/2.2.0/gems/asciidoctor-1.5.2/lib/asciidoctor/cli/invoker.rb:86:in `invoke!'
        from C:/Ruby22/lib/ruby/gems/2.2.0/gems/asciidoctor-1.5.2/bin/asciidoctor:10:in `<top (required)>'
        from C:/Ruby22/bin/asciidoctor:23:in `load'
        from C:/Ruby22/bin/asciidoctor:23:in `<main>'

I've tried all manner of quotes and escapes and both ways slashes, and still the error

I'm converting from asciidoc, and this works without error in asciidoc.

Is there some argument to asciidoctor to tell it not to try to create directories for css files?

Am I doing something wrong?