Ver. 0.8.67 ERROR: Unhandled exception

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\convert\single.py", line 252, in accept
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'


I'm trying to convert an epub or a mobi into a pdf but I always get that error.
Can someone please tell me what it means and how to fix it? It only happened after I installed the latest version.
Thank you.