Strange error in jcode.rb when converting range to array

In my rails app I've line with the following code:

("a".."z").to_a

Simple enough and it's been working for a while. Last night this line
started throwing the following error:

undefined method `[]' for nil:NilClass
/usr/local/lib/ruby/1.8/jcode.rb:83:in `succ!'
/usr/local/lib/ruby/1.8/jcode.rb:94

I looked up jcode.rb and the comments for this file say that it's "#
jcode.rb - ruby code to handle japanese (EUC/SJIS) string"

This is the part of jcode.rb that throws the error (line 83)

78 def succ!
79 reg = end_regexp
80 if self =~ reg
81 succ_table = SUCC[$KCODE[0,1].downcase]
83 begin
84 self[-1] += succ_table[self[-1]]
85 self[-2] += 1 if self[-1] == 0
86 end while self !~ reg
87 self
88 else
89 original_succ!
90 end
91 end

Debugging is difficult because if I restart the app, the problem goes
away..for a while. Has anyone else encountered this error before?

It's obviously being caused by something I've added to my app
recently. I just can't figure out what.

ruby -v
ruby 1.8.4 (2005-12-24) [i686-darwin8.5.2]

Thanks

Hammed