asciidoctor-pdf: fresh install of ruby and asciidoctor-pdf does not work

Have installed everything on a virgin windows PC using gem. Something seems to be missing…

>ruby --version
ruby 2.2.6p396 (2016-11-15 revision 56800) [i386-mingw32]

>asciidoctor-pdf -v
C:/Ruby22/lib/ruby/gems/2.2.0/gems/prawn-templates-0.0.3/lib/prawn/templates.rb:246:in `<top (required)>': can't modify frozen Array (RuntimeError)
    from C:/Ruby22/lib/ruby/2.2.0/rubygems/core_ext/kernel_require.rb:128:in `require'
    from C:/Ruby22/lib/ruby/2.2.0/rubygems/core_ext/kernel_require.rb:128:in `rescue in require'
    from C:/Ruby22/lib/ruby/2.2.0/rubygems/core_ext/kernel_require.rb:39:in `require'
    from C:/Ruby22/lib/ruby/gems/2.2.0/gems/asciidoctor-pdf-1.5.0.alpha.14/lib/asciidoctor-pdf/converter.rb:10:in `<top (required)>'
    from C:/Ruby22/lib/ruby/gems/2.2.0/gems/asciidoctor-pdf-1.5.0.alpha.14/lib/asciidoctor-pdf.rb:3:in `require_relative'
    from C:/Ruby22/lib/ruby/gems/2.2.0/gems/asciidoctor-pdf-1.5.0.alpha.14/lib/asciidoctor-pdf.rb:3:in `<top (required)>'
    from C:/Ruby22/lib/ruby/2.2.0/rubygems/core_ext/kernel_require.rb:54:in `require'
    from C:/Ruby22/lib/ruby/2.2.0/rubygems/core_ext/kernel_require.rb:54:in `require'
    from C:/Ruby22/lib/ruby/gems/2.2.0/gems/asciidoctor-pdf-1.5.0.alpha.14/bin/asciidoctor-pdf:4:in `<top (required)>'
    from C:/Ruby22/bin/asciidoctor-pdf:23:in `load'
    from C:/Ruby22/bin/asciidoctor-pdf:23:in `<main>'