Editing TOC get ValueError: invalid literal for int() with base 10: ''

When I add an entry in editing ToC, an error message pops up:

ValueError: invalid literal for int() with base 10: ''
This only happens to a specific epub book, which is in the attachment.

I'm using the latest version, v5.13 x64, on windows 10.

Could anyone help?

Attached Files

problematic book.epub (4.31 MB)