rbx bug in IO#lineno

gistfile1.rb

counter={}
 File.open('lines.txt', 'r') do |f|
   f.each_line do |l|
       counter[f.lineno] = l
   end
 end

counter.each_pair {|k,v| puts "#{k}: #{v}"}

#where lines.txt is
#1
#2
#3
#4
#5
#6
#7
#8