Uncaught ReferenceError: pagenow is not defined

Am using the latest version of Simple Fields, and trying to use Post Thumbnail Editor at the same time. However, when I try to change an image’s thumbnail size the pop up is just loading and nothing happens.
The error log gives me the following error message:

Uncaught ReferenceError: pagenow is not defined

in scripts.js:623

http://wordpress.org/extend/plugins/simple-fields/