OS X :: Cannot Add New Desktop Background Folder

In the Desktop & Screen Saver pane of System Preferences, I wanted to add a new folder for use as my desktop background images. However, when I click on the + icon to add the new folder, it does not appear in the source list and instead, it duplicates my Aperture library source, as seen in attachment 1. If I try to add the new folder again, it says it is already present in the source list, as seen in attachment 2.

I've tried deleting my com.apple.desktop.plist preference file, and that resets things, but when I go to add the folder, it does the same thing. I've tried different folder locations, different folder names -- nothing works.