Error when converting a PDF to any format

For Calibre 3.20.0 on W10 (also happening on Ubuntu):

Converting a 8.8Mb PDF to AZW3 (or MOBI) I get the following error:

... truncating top of log file

Creating KF8 output
Generating in-line ToC
Generating KF8 markup...
Compressing markup...
Creating indices...
Python function terminated unexpectedly
'H' format requires 0 <= number <= 65535 (Error Code: 1)
Traceback (most recent call last):
File "site.py", line 101, in main
File "site.py", line 78, in run_entry_point
File "site-packages\calibre\utils\ipc\worker.py", line 195, in main
File "site-packages\calibre\gui2\convert\gui_conversion.py", line 33, in gui_convert_override
File "site-packages\calibre\gui2\convert\gui_conversion.py", line 26, in gui_convert
File "site-packages\calibre\ebooks\conversion\plumber.py", line 1252, in run
File "site-packages\calibre\ebooks\conversion\plugins\mobi_ou tput.py", line 329, in convert
File "site-packages\calibre\ebooks\mobi\writer8\mobi.py", line 342, in write
struct.error: 'H' format requires 0 <= number <= 65535

It took almost 4 hours to process this 8.8Mb file, and produced over 100,000 1K blank images in each of the 4 log directories (approx. 450,000 1K images). So I'd call that a bug somewhere.