[EldritchMath]

I'm trying to use Calibre to modify a set of books (the Dresden Files in .epub format) to be grammatically and just generally correct, as I'm a bit OCD about that sort of thing. However, when modifying "Fool Moon" in particular, it will work as normal up until index_split_007 or so, at which point, when I try to open the next index split, it gives the following error:

"calibre, version 3.13.0 ERROR: Failed to open book: Failed to open book, click "Show details" for more information.

Traceback (most recent call last): File "site-packages\calibre\gui2\tweakbook\job.py", line 35, in run File "site-packages\calibre\gui2\tweak_book\boss.py", line 59, in get_container File "site-packages\calibre\ebooks\oeb\polish\container.py", line 1497, in get_container File "site-packages\calibre\ebooks\oeb\polish\container.py", line 1155, in __init_ InvalidEpub: No META-INF/container.xml in epub"

What could consistently cause this single error? It only has happened on this one book so far, and I'm in the process of editing the fourth book, so I thought it might have something to do with the file itself. However, it sometimes changes when it takes effect: the first time, it happened after index_split_009, but the most recent time, it happened after index_split_007. What could cause this? Is there any way to universally prevent it? Even better, is there a way to fix the file now, so I don't lose all of my progress in editing? Up until now, I've just had to delete the book and try to work on it again. Any help would be appreciated!