error on merging books

[L LaBella]

calibre 7.13
Windows 11 23H2

example files attached

attempting to merge 2 books, for instance epub and azw3 versions. merge fails with "TypeError:cannot unpack non-iterable bool object"

details:

calibre, version 7.13.0
ERROR: Unhandled exception: <b>TypeError</b>:cannot unpack non-iterable bool object

calibre 7.13 embedded-python: True
Windows-10-10.0.22631-SP0 Windows ('64bit', 'WindowsPE')
('Windows', '10', '10.0.22631')
Python 3.11.5
Windows: ('10', '10.0.22631', 'SP0', 'Multiprocessor Free')
Interface language: None
EXE path: C:\Program Files\Calibre2\calibre.exe
Successfully initialized third party plugins: Gather KFX-ZIP (from KFX Input) (2, 15, 0) && DeDRM (10, 0, 3) && Package KFX (from KFX Input) (2, 15, 0) && Action Chains (1, 20, 7) && Baen (1, 1, 0) && Barnes & Noble (1, 5, 5) && Comments Cleaner (1, 13, 0) && Count Pages (1, 13, 6) && EpubMerge (2, 18, 0) && Extract ISBN (1, 6, 3) && FanFicFare (4, 35, 0) && Fantastic Fiction (1, 7, 0) && Fantastic Fiction Adults (1, 3, 1) && FictionDB (1, 4, 2) && Find Duplicates (1, 10, 9) && Generate Cover (2, 3, 4) && Goodreads (1, 8, 2) && ISFDB3 (1, 4, 0) && Import List (1, 9, 4) && KFX metadata reader (from KFX Input) (2, 15, 0) && From KFX (2, 15, 0) && KFX Input (2, 15, 0) && Kobo Books (1, 9, 2) && Manage Series (1, 5, 2) && Markdown Output (1, 0, 3) && Modify ePub (1, 8, 3) && Obok DeDRM (10, 0, 3) && Open With (1, 8, 3) && Quality Check (1, 13, 11) && Search The Internet (1, 11, 2) && Smashwords Metadata (1, 0, 2) && Standard Ebooks (1, 0, 0) && TTS to MP3 (0, 6, 0) && View Manager (1, 10, 3) && Wiki Reader (2, 2, 1)
Traceback (most recent call last):
  File "calibre\gui2\actions\edit_metadata.py", line 644, in merge_books
TypeError: cannot unpack non-iterable bool object