why this code won't run on ruby 1.8.7? (it runs on 1.9.3)

gistfile1.txt

require 'pp'

class Bar
  attr_accessor :foo_bars
  def initialize(&block)
    @foo_bars = []
    yield self
  end

  def foo_bar(&block)
    @foo_bars.push(block)
  end
  
  def run(x)
    @foo_bars.each do
      |foo_bar|
      pp foo_bar.call(x)
    end
  end
end

pp Bar.new {
  |b|
  b.foo_bar do
    |x|
    if x.nil?
      'x is empty!'
    else
      "x is #{x}"
    end
  end

  b.foo_bar do
    |x|
    x.gsub(/^(.)/,'f\1')
  end
}.run("0")