ePub Doesn't work with Google Play Books

I tried uploading to Google Play Books and it says "file cannot be processed." then has a "learn more" option taking me to this page: https://support.google.com/googleplay/answer/3097151?visit_id=636829228570910203-3019479125&rd=1

Something about the formatting (I assume) of the ePub isn't allowing it to be added there. Would be nice if it worked.