Build something that works
Check the Notes - the paperclip gem was updated since the video was recorded, and there's an extra line you need to add to your listing.rb file. Just as an FYI - the Transcript/Notes will always be up-to-date and will sometimes have helpful info that doesn't appear in the video. I recommend giving them a quick scan after each video.
In the future, please post any troubleshooting-related questions in the 'Discussion' section beneath the video - thanks!