Differing output for function moved from ruby 1.8 to 1.9

Sup, fools?

This is the Levenshtein function I'm gankin' for my file comparison
project (see "40 million comparison..." thread):

# Levenshtein calculator
# Author: Paul Battley (pbattley <at> gmail.com)
# Modified slightly by John Perkins:
# -- removed $KCODE call

def distance(str1, str2)

  unpack_rule = 'C*'
  s = str1.unpack(unpack_rule)
  t = str2.unpack(unpack_rule)
  n = s.length
  m = t.length

  return m if (0 == n) # stop the madness if either string is empty
  return n if (0 == m)

  d = (0..m).to_a
  x = nil

  (0...n).each do |i|
    e = i + 1
    (0...m).each do |j|
      cost = (s[i] == t[j]) ? 0 : 1
      x = [
        d[j + 1] + 1,   # insertion