Calibre 2.68 conversion from docx to epub failed

I'm using the latest version of Calibre on Windows 10, trying to convert a docx file to epub. But the conversion always fails with an error message that I can't figure out.

TypeError: make_block() argument after * must be a sequence, not NoneType

Any help or suggestions please?
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InputFormatPlugin: DOCX Input running
on C:\Users\AAA\AppData\Local\Temp\calibre_vvhwr3\hjd o6_.docx
Python function terminated unexpectedly
make_block() argument after * must be a sequence, not NoneType (Error Code: 1)
Traceback (most recent call last):
File "site.py", line 132, in main
File "site.py", line 109, in run_entry_point
File "site-packages\calibre\utils\ipc\worker.py", line 190, in main
File "site-packages\calibre\gui2\convert\gui_conversion.py", line 31, in gui_convert_override
File "site-packages\calibre\gui2\convert\gui_conversion.py", line 25, in gui_convert
File "site-packages\calibre\ebooks\conversion\plumber.py", line 1060, in run
File "site-packages\calibre\customize\conversion.py", line 241, in __call__
File "site-packages\calibre\ebooks\conversion\plugins\docx_in put.py", line 33, in convert
File "site-packages\calibre\ebooks\docx\to_html.py", line 98, in __call__
File "site-packages\calibre\ebooks\docx\fields.py", line 144, in __call__
File "site-packages\calibre\ebooks\docx\fields.py", line 212, in parse_index
File "site-packages\calibre\ebooks\docx\index.py", line 109, in process_index
TypeError: make_block() argument after * must be a sequence, not NoneType
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