Mechanize TypeError: can't convert nil into String

Problem

>> page = agent.submit(login_form)
TypeError: can't convert nil into String
        from /usr/lib/ruby/gems/1.8/gems/mechanize-0.9.3/lib/www/mechanize/util.rb:40:in `iconv'
        from /usr/lib/ruby/gems/1.8/gems/mechanize-0.9.3/lib/www/mechanize/util.rb:40:in `from_native_charset'
        from /usr/lib/ruby/gems/1.8/gems/mechanize-0.9.3/lib/www/mechanize/form.rb:152:in `from_native_charset'
        from /usr/lib/ruby/gems/1.8/gems/mechanize-0.9.3/lib/www/mechanize/form.rb:144:in `proc_query'
        from /usr/lib/ruby/gems/1.8/gems/mechanize-0.9.3/lib/www/mechanize/form.rb:143:in `map'
        from /usr/lib/ruby/gems/1.8/gems/mechanize-0.9.3/lib/www/mechanize/form.rb:143:in `proc_query'
        from /usr/lib/ruby/gems/1.8/gems/mechanize-0.9.3/lib/www/mechanize/form.rb:166:in `build_query'
        from /usr/lib/ruby/gems/1.8/gems/mechanize-0.9.3/lib/www/mechanize/form.rb:165:in `each'
        from /usr/lib/ruby/gems/1.8/gems/mechanize-0.9.3/lib/www/mechanize/form.rb:165:in `build_query'
        from /usr/lib/ruby/gems/1.8/gems/mechanize-0.9.3/lib/www/mechanize/form.rb:214:in `request_data'
        from /usr/lib/ruby/gems/1.8/gems/mechanize-0.9.3/lib/www/mechanize.rb:401:in `post_form'
        from /usr/lib/ruby/gems/1.8/gems/mechanize-0.9.3/lib/www/mechanize.rb:344:in `submit'

Patch(Line 40)

root@khojguru:~# vi /usr/lib/ruby/gems/1.8/gems/mechanize-0.9.3/lib/www/mechanize/util.rb
...
def from_native_charset(s, code)
  if Mechanize.html_parser == Nokogiri::HTML
    return unless s
    - Iconv.iconv(code, "UTF-8", s).join("")
    + Iconv.iconv(code.to_s, "UTF-8", s.to_s).join("")
  else
    return s
  end
end
...