with defusedxml installed, mat2 on a simple .docx fails

now that i have python3-defusedxml installed, i get a failure with mat2 on a simple hello-world.docx

0 dkg@alice:~/src/mat$ mat2 hello-world.docx 
multiprocessing.pool.RemoteTraceback: 
"""
Traceback (most recent call last):
  File "/usr/lib/python3.6/multiprocessing/pool.py", line 119, in worker
    result = (True, func(*args, **kwds))
  File "/home/dkg/src/mat/mat2/mat2", line 85, in clean_meta
    return p.remove_all()
  File "/home/dkg/src/mat/mat2/libmat2/office.py", line 100, in remove_all
    if self._specific_cleanup(full_path) is False:
  File "/home/dkg/src/mat/mat2/libmat2/office.py", line 210, in _specific_cleanup
    return self.__remove_revisions(full_path)
  File "/home/dkg/src/mat/mat2/libmat2/office.py", line 173, in __remove_revisions
    tree, namespace = _parse_xml(full_path)
  File "/home/dkg/src/mat/mat2/libmat2/office.py", line 28, in _parse_xml
    ET.register_namespace(key, value)
AttributeError: module 'defusedxml.ElementTree' has no attribute 'register_namespace'
"""

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/home/dkg/src/mat/mat2/mat2", line 154, in <module>
    sys.exit(main())
  File "/home/dkg/src/mat/mat2/mat2", line 150, in main
    ret = list(p.imap_unordered(clean_meta, list(l)))
  File "/usr/lib/python3.6/multiprocessing/pool.py", line 735, in next
    raise value
AttributeError: module 'defusedxml.ElementTree' has no attribute 'register_namespace'
1 dkg@alice:~/src/mat$