The bug causes an error form Nokogiri's 1.9 mode. Also, the bug is reported in issue#657, https://github.com/sparklemotion/nokogiri/issues/657 .
Please look at seplist method in lib/ruby/1.9/pp.rb, line 214-225. A given argument as "list" looks like this:
This turns out *v should be [:name, "html"] . In seplist method, the value of *v is a correct one, [:name, "html"]. However, in the given block, (the first) argument is :name only. The "yield" method, line#223, is suspicious.
Nokogiri uses seplist method in lib/nokogiri/xml/pp/node.rb, https://github.com/sparklemotion/nokogiri/blob/master/lib/nokogiri/xml/pp/node.rb, line#36-48.
On JRuby 1.9, "v" in Nokogiri's pretty_print method, line#37 in https://github.com/sparklemotion/nokogiri/blob/master/lib/nokogiri/xml/pp/node.rb , isn't an Array and doesn't have first method. Then, an error is thrown from here.