Unable to View Kepub

[HogDrvr]

I have converted a bunch of epbub and azw3 files to Kepub but now I am unable to view them using the internal Calibre viewer. I get the following error output:

calibre, version 7.26.0
ERROR: Loading book failed: Failed to open the book at /Users/ed/Ebooks/Adrian R. Magnuson/Taking Flight (2219)/Taking Flight - Adrian R. Magnuson.kepub. Click "Show details" for more info.

Failed to convert book: /Users/ed/Ebooks/Adrian R. Magnuson/Taking Flight (2219)/Taking Flight - Adrian R. Magnuson.kepub with error:
Failed to run pipe worker with command: from calibre.srv.render_book import viewer_main; viewer_main()
Traceback (most recent call last):
File "runpy.py", line 198, in _run_module_as_main
File "runpy.py", line 88, in _run_code
File "site.py", line 42, in <module>
File "site.py", line 38, in main
File "calibre/utils/ipc/worker.py", line 196, in main
File "<string>", line 1, in <module>
File "calibre/srv/render_book.py", line 845, in viewer_main
File "calibre/srv/render_book.py", line 836, in render_for_viewer
File "calibre/srv/render_book.py", line 813, in render
File "calibre/ebooks/oeb/iterator/book.py", line 55, in extract_book
File "calibre/ebooks/conversion/plumber.py", line 771, in __init__
ValueError: No plugin to handle input format: kepub

[DNSB]

At this time, calibre does not display .kepub files. The file extension used by Kobo ereaders is .kepub.epub which calibre happily displays as an ePub. When calibre sends a .kepub to a Kobo ereader it adds .epub to create that double barrelled file extension.

One other clue might be the icon used by calibre for a .kepub file which is a page with a question mark.

Edit: Not absolutely certain but I think that 7.26.104 will display a .kepub but haven't tested that.