Cannot add additional formats via drag-and-drop in 5.8.1

[A M]

Since upgrading to 5.8.1 it is no longer possible to add additional book formats via drag-and-drop from a Windows Explorer window onto the informational side panel of a selected book in the Calibre library book list. It is expected that doing so would add the additional format to the book that is selected, however, what happens now is the following error:

calibre, version 5.8.1
ERROR: Unhandled exception: <b>TypeError</b>:cannot unpack non-iterable bool object

calibre 5.8.1 [64bit] embedded-python: True is64bit: True
Windows-10-10.0.19041 Windows ('64bit', 'WindowsPE')
('Windows', '10', '10.0.19041')
Python 3.8.5
Windows: ('10', '10.0.19041', '', 'Multiprocessor Free')
Interface language: None
Traceback (most recent call last):
  File "calibre\gui2\actions\add.py", line 455, in files_dropped_on_book
TypeError: cannot unpack non-iterable bool object

The current workarounds are either to (1) Edit [book] metadata individually and copy the additional format book file into the filetype list in the top right corner, or (2) add a duplicate book with the additional format book file and then merge the books together.

[Kovid Goyal (kovid)]

Works for me on my windows machine. Are you dragging one file or
multiple? What format? What did you click in the popup asking for
confirmation? If you didnt get a popup try going to
Preferences->behavior->Reset all disabled confirmation dialogs

[Kovid Goyal (kovid)]

Fixed in branch master. The fix will be in the next release. calibre is usually released every alternate Friday.

 status fixreleased