为什么在Rake任务中调用字符串上的to_a无效,但在Runner脚本中完全有效？

require 'openssl'

KEY = 'secretkey'

  namespace :import do
  task :users => :environment do
      def decrypt_password(pw)

          cipher = OpenSSL::Cipher::Cipher.new('bf-ecb')
          cipher.decrypt
          cipher.key = KEY.to_a.pack('H*')    <<--------- FAILS RIGHT HERE on to_a

          data = data.to_a.pack('H*')
          data = cipher.update(data)
          data << cipher.final
          unpad(data)

      end
   end

   ... other methods
end

(Rails 3.0.0,Ruby 1.9.2)