One thing to look out for though is the download site has ads which try to get you to download random junk -- the correct download is a zipfile.
http://Adblockplus.org is your friend.
But this error message does not imply a DRM lock. Usually DRM results in a "DRMError".
Sounds like the book is malformed/corrupted or has weird code. I am sure Kovid will come by and explain the error message with hopefully a fix.

EDIT: Yep.