Error message when sending to device (Kobo Glo HD)

Today I tried to send a number of EPUB books to my Kobo GloHD. Most of them went without a hitch, but four of them threw up an error message. All of the books that had an error were purchased from Kobo in 2014 and 2016, but many of the ones that did send OK were also.

I've copied the error dialog for one of them here. Any insights on what might be happening and how to fix?


calibre, version 5.23.0
ERROR: Error: Error communicating with device

Failed to process Black Sheep by Georgette Heyer: list index out of range

Traceback (most recent call last):
File "calibre_plugins.kobotouch_extended.device.driver" , line 270, in _modify_epub
common.modify_epub(
File "calibre_plugins.kobotouch_extended.common", line 236, in modify_epub
container.convert()
File "calibre_plugins.kobotouch_extended.container" , line 384, in convert
self.__run_async_over_content(self.add_kobo_spans)
File "calibre_plugins.kobotouch_extended.container" , line 380, in __run_async_over_content
self.__run_async(func, names)
File "calibre_plugins.kobotouch_extended.container" , line 373, in __run_async
future.result(timeout=10)
File "concurrent/futures/_base.py", line 432, in result
File "concurrent/futures/_base.py", line 388, in __get_result
File "concurrent/futures/thread.py", line 57, in run
File "calibre_plugins.kobotouch_extended.container" , line 506, in add_kobo_spans
body = root.xpath("./xhtml:body", namespaces={"xhtml": XHTML_NAMESPACE})[0]
IndexError: list index out of range

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
File "calibre/gui2/device.py", line 89, in run
File "calibre/gui2/device.py", line 595, in _upload_books
File "calibre_plugins.kobotouch_extended.device.driver" , line 400, in upload_books
return super(KOBOTOUCHEXTENDED, self).upload_books(
File "calibre/devices/kobo/driver.py", line 2137, in upload_books
File "calibre_plugins.kobotouch_extended.device.driver" , line 296, in _modify_epub
raise e.__class__(msg).with_traceback(tb)
File "calibre_plugins.kobotouch_extended.device.driver" , line 270, in _modify_epub
common.modify_epub(
File "calibre_plugins.kobotouch_extended.common", line 236, in modify_epub
container.convert()
File "calibre_plugins.kobotouch_extended.container" , line 384, in convert
self.__run_async_over_content(self.add_kobo_spans)
File "calibre_plugins.kobotouch_extended.container" , line 380, in __run_async_over_content
self.__run_async(func, names)
File "calibre_plugins.kobotouch_extended.container" , line 373, in __run_async
future.result(timeout=10)
File "concurrent/futures/_base.py", line 432, in result
File "concurrent/futures/_base.py", line 388, in __get_result
File "concurrent/futures/thread.py", line 57, in run
File "calibre_plugins.kobotouch_extended.container" , line 506, in add_kobo_spans
body = root.xpath("./xhtml:body", namespaces={"xhtml": XHTML_NAMESPACE})[0]
IndexError: Failed to process Black Sheep by Georgette Heyer: list index out of range