Error setting MOBI output to 'new'

[Seán Ó Séaghdha]

I tried changing the MOBI file type to 'new' in the Output Options preferences dialog and got this error after clicking the Apply button. As far as I can tell the setting is changed - i.e the 'old' format opens on the Kindle Touch, but the 'new' does not.

calibre, version 0.8.67
ERROR: Unhandled exception: <b>TypeError</b>:arguments did not match any overloaded call:
  QFontInfo(QFont): argument 1 has unexpected type 'QString'
  QFontInfo(QFontInfo): argument 1 has unexpected type 'QString'

Traceback (most recent call last):
  File "site-packages\calibre\gui2\preferences\main.py", line 334, in commit
  File "site-packages\calibre\gui2\preferences\conversion.py", line 80, in commit
  File "site-packages\calibre\gui2\convert\__init__.py", line 280, in commit
  File "site-packages\calibre\gui2\convert\__init__.py", line 107, in commit_options
  File "site-packages\calibre\gui2\convert\__init__.py", line 117, in create_recommendations
  File "site-packages\calibre\gui2\convert\__init__.py", line 146, in get_value
TypeError: arguments did not match any overloaded call:
  QFontInfo(QFont): argument 1 has unexpected type 'QString'
  QFontInfo(QFontInfo): argument 1 has unexpected type 'QString'

Windows 7 Enterprise 64-bit