Thin won't load gem tzinfo

I got a rails application with a file in my lib directory containing:

require 'tzinfo'

module TimeConvert
  @@acst = TZInfo::Timezone.get('Australia/Darwin')
  @@aest = TZInfo::Timezone.get('Australia/Sydney')

  def self.acst_to_aest(time)
      @@aest.utc_to_local(@@acst.local_to_utc(time))
  end

end

I got a model in my rails application which includes the module TimeConvert.

When I launch my rails application using thin (thin start), when reaching a page using this model, I get the error

no such file to load -- tzinfo

If I launch my rails application using mongrel (with script/server), my application works.

I don't know what the problem is at all. But it seems the bug is due to Thin.