ERROR: Could not open ebook: Unknown book type

[wooboo]

Hello and thanks in advance for any assistance with this error.

It seems that a batch of ebooks have headed south into unknown book type territory. I'm not sure if it's something I did, or if the books were corrupted to begin with. I've double checked the input/output formats, ran the plug in "Quality Check", etc.

I did try opening one of the unknown book types with mobipocket reader and it wouldn't open.

1) What else can I check/double-check on the error?

2) How can I quickly find what books are "unknown"? Opening each one to test will just take too long, hoping there's a faster way.

Additional info:
calibre, version 0.8.50
ERROR: Could not open ebook: Unknown book type: '\x00\x00\x00\x00\x00\x00\x00\x00'

Traceback (most recent call last):
File "site-packages\calibre\gui2\viewer\main.py", line 63, in run
File "threading.py", line 483, in run
File "site-packages\calibre\ebooks\oeb\iterator.py", line 207, in __enter__
File "site-packages\calibre\customize\conversion.py", line 208, in __call__
File "site-packages\calibre\ebooks\conversion\plugins\mobi_in put.py", line 32, in convert
File "site-packages\calibre\ebooks\mobi\reader\mobi6.py", line 83, in __init__
MobiError: Unknown book type: '\x00\x00\x00\x00\x00\x00\x00\x00'


Thanks again.