"AttributeError: 'NoneType' object has no attribute 'execute'" if firefox is not installed

[Markus Korn]

I get this Traceback if firefox is not installed (a history db is not found):

DEBUG:root:Updating database with new Firefox History items
Traceback (most recent call last):
  File "/home/markus/zeitgeist/trunk/zeitgeist/../zeitgeist-datahub", line 111, in _update_db_async
    for num, item in enumerate(self._sources_queue[0].get_items()):
  File "/home/markus/zeitgeist/trunk/zeitgeist/../_zeitgeist/loggers/zeitgeist_base.py", line 107, in _wrapper
    for n, i in enumerate(self.get_items_uncached()):
  File "/home/markus/zeitgeist/trunk/zeitgeist/../_zeitgeist/loggers/datasources/firefox.py", line 159, in get_items_uncached
    history = self.cursor.execute(
AttributeError: 'NoneType' object has no attribute 'execute'

[heartsmagic (heartsmagic)]

I can confirm this error from a fresh installed zeitgeist via PPA: https://launchpad.net/~zeitgeist/+archive/ppa/
I have Firefox installed.