Export Layers to PDF

I get the following error while trying to run the script:
TypeError: 'NoneType' object has no attribute '__getitem__'

I saved the script here: C:\Users\Taylor\.gimp-2.8\plug-ins\export-layers-to-pdf.py

I installed ImageMagick, and edited the default.env file as follows:

# Example entry in files like these
# FOOPATH=/path/to/foo/stuff
PATH=${gimp_installation_dir}\bin;C:\Program Files\ImageMagick-6.8.4-Q16

I am running Windows 8 with Gimp 2.8.4. Any ideas what I am doing wrong, and how to correct it?
Thanks,
Taylor