IndexError: undefined group name reference: year
/Users/me/.rbenv/versions/2.4.3/lib/ruby/gems/2.4.0/gems/asciidoctor-iso-0.9.3/lib/asciidoctor/iso/ref.rb:70:in `[]'
/Users/me/.rbenv/versions/2.4.3/lib/ruby/gems/2.4.0/gems/asciidoctor-iso-0.9.3/lib/asciidoctor/iso/ref.rb:70:in `isorefmatches3'