Zip folder blank after downloading

[amac9]

I have successfully used the extension on multiple books, however one book (technically a collection of 5 small books in a series) I tried recently appears to work correctly, creates the zip file, and shows up in my downloads folder with a size of 146mb. When I try to unzip the folder it says it is empty.

I've tried deleting my cache, restarting firefox, refreshing firefox, and removing/adding the extension. Any ideas on what to try next?