Polish books fails

[desertdaisy]

Running the Polish books plugin on an .epub that seems like all the others, but the process fails.
Here's the error message:

"calibre, version 3.39.1 (darwin, embedded-python: True)
Conversion error: Failed: Polish Neither Here Nor There

Polish Neither Here Nor There
Python function terminated unexpectedly: Failed to find obfuscation key
Traceback (most recent call last):
File "/Applications/calibre.app/Contents/Resources/Python/lib/python2.7/site.py", line 154, in main
return run_entry_point()
File "/Applications/calibre.app/Contents/Resources/Python/lib/python2.7/site.py", line 114, in run_entry_point
return getattr(pmod, func)()
File "site-packages/calibre/utils/ipc/worker.py", line 199, in main
File "site-packages/calibre/ebooks/oeb/polish/main.py", line 270, in gui_polish
File "site-packages/calibre/ebooks/oeb/polish/main.py", line 247, in polish
File "site-packages/calibre/ebooks/oeb/polish/container.py", line 1519, in get_container
File "site-packages/calibre/ebooks/oeb/polish/container.py", line 1184, in __init__
File "site-packages/calibre/ebooks/oeb/polish/container.py", line 1301, in process_encryption
calibre.ebooks.oeb.polish.container.ObfuscationKey Missing: Failed to find obfuscation key
"

[kovidgoyal]

That EPUB is invalid, it is marked as being DRMed but is missing the encryption key. You can try doing an EPUB to EPUB conversion instead.