Merging books is no longer possible

[SandFish]

This happened with update to calibre 7.13 with the following error message:

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: D:\Program Files\Calibre2\calibre.exe
Successfully initialized third party plugins: Gather KFX-ZIP (from KFX Input) (2, 15, 0) && Package KFX (from KFX Input) (2, 15, 0) && Count Pages (1, 13, 6) && DNB_DE (3, 2, 4) && Diaps Editing Toolbag (0, 4, 0) && Ebook Translator (2, 3, 5) && EpubCheck (0, 2, 5) && Extract ISBN (1, 6, 3) && Fantastic Fiction (1, 7, 0) && Fantastic Fiction Adults (1, 3, 1) && Find Duplicates (1, 10, 9) && Generate Cover (2, 3, 4) && KFX metadata reader (from KFX Input) (2, 15, 0) && From KFX (2, 15, 0) && KFX Input (2, 15, 0) && Set KFX metadata (from KFX Output) (2, 8, 0) && KFX Output (2, 8, 0) && Perrypedia (1, 9, 0) && Quality Check (1, 13, 11) && Reformat plugin (2, 4, 0) && Resize Cover (1, 2, 1) && Unplugged (1, 0, 0)
Traceback (most recent call last):
  File "calibre\gui2\actions\edit_metadata.py", line 644, in merge_books
TypeError: cannot unpack non-iterable bool object

[Charles Haley (cbhaley)]

@Kovid: perhaps you might make a preview to provide the fix? That wouldn't stop the continuous duplicate bug reports but it would offer a solution.

[Kovid Goyal (kovid)]

There is certainly a lot more merging with turned off confirmation going
on than I expected. As for a fix, merging via drag and drop will still
work. And the preview release will happen in a few days.