rmagick error on heroku

Question

i'm cropping images with paperclip.

i have a custom module which works great on my local machine (copied from railscast 182).

//file: lib/paperclip_processors/cropper.rb
module Paperclip
  class Cropper < Thumbnail
    def transformation_command
      if crop_command
        crop_command + super.sub(/ -crop \S+/, '')
      else
        super
      end
    end 

    def crop_command
      target = @attachment.instance
      if target.cropping?
        " -crop '#{target.crop_w.to_i}x#{target.crop_h.to_i}+#{target.crop_x.to_i}+#    {target.crop_y.to_i}'"
      end
    end
  end
end

on heroku i get the following error:

NoMethodError (private method `sub' called for ["-resize", "220x", "-crop", "220x220+0+18", "+repage"]:Array):

lib/paperclip_processors/cropper.rb:12:in `transformation_command'

paperclip (2.3.3) lib/paperclip/thumbnail.rb:55:in `make'

paperclip (2.3.3) lib/paperclip/processor.rb:33:in `make'

paperclip (2.3.3) lib/paperclip/attachment.rb:295:in `post_process_styles'

paperclip (2.3.3) lib/paperclip/attachment.rb:294:in `each'

paperclip (2.3.3) lib/paperclip/attachment.rb:294:in `inject'

paperclip (2.3.3) lib/paperclip/attachment.rb:294:in `post_process_styles'

paperclip (2.3.3) lib/paperclip/attachment.rb:291:in `each'

paperclip (2.3.3) lib/paperclip/attachment.rb:291:in `post_process_styles'

paperclip (2.3.3) lib/paperclip/attachment.rb:285:in `post_process'

paperclip (2.3.3) lib/paperclip/callback_compatability.rb:23:in `call'

paperclip (2.3.3) lib/paperclip/callback_compatability.rb:23:in `run_paperclip_callbacks'

paperclip (2.3.3) lib/paperclip/attachment.rb:284:in `post_process'

paperclip (2.3.3) lib/paperclip/callback_compatability.rb:23:in `call'

paperclip (2.3.3) lib/paperclip/callback_compatability.rb:23:in `run_paperclip_callbacks'

paperclip (2.3.3) lib/paperclip/attachment.rb:283:in `post_process'

paperclip (2.3.3) lib/paperclip/attachment.rb:214:in `reprocess!'

app/models/user.rb:339:in `reprocess_avatar'

app/controllers/user_controller.rb:57:in `update_avatar'

haml (2.2.3) rails/./lib/sass/plugin/rails.rb:19:in `process'

/home/heroku_rack/lib/static_assets.rb:9:in `call'

/home/heroku_rack/lib/last_access.rb:25:in `call'

/home/heroku_rack/lib/date_header.rb:14:in `call'

thin (1.0.1) lib/thin/connection.rb:80:in `pre_process'

thin (1.0.1) lib/thin/connection.rb:78:in `catch'

thin (1.0.1) lib/thin/connection.rb:78:in `pre_process'

thin (1.0.1) lib/thin/connection.rb:57:in `process'

thin (1.0.1) lib/thin/connection.rb:42:in `receive_data'

eventmachine (0.12.6) lib/eventmachine.rb:240:in `run_machine'

eventmachine (0.12.6) lib/eventmachine.rb:240:in `run'

thin (1.0.1) lib/thin/backends/base.rb:57:in `start'

thin (1.0.1) lib/thin/server.rb:150:in `start'

thin (1.0.1) lib/thin/controllers/controller.rb:80:in `start'

thin (1.0.1) lib/thin/runner.rb:173:in `send'

thin (1.0.1) lib/thin/runner.rb:173:in `run_command'

thin (1.0.1) lib/thin/runner.rb:139:in `run!'

thin (1.0.1) bin/thin:6

/usr/local/bin/thin:20:in `load'

/usr/local/bin/thin:20