With rails 2.0.2 and ruby 1.8.7 you can find this error using rails truncate helper

To solve this problem, paste this code in your enviroment.rb (eof).

module ActionView
  module Helpers
    module TextHelper
      def truncate(text, length = 30, truncate_string = "...")
        if text.nil? then return end
        l = length - truncate_string.chars.to_a.size
        (text.chars.to_a.size > length ? text.chars.to_a[0...l].join + truncate_string : text).to_s
      end
    end
  end
end

Have a nice day!