Crash when checking free space on upgrade from edgy to feisty

Binary package hint: update-manager

Today I tried upgrading from edgy to feisty using update-manager (0.57.2), but it failed with the following traceback:

Traceback (most recent call last):

  File "/tmp/tmpnkDe9U/feisty", line 47, in ?
    app.run()

  File "/tmp/tmpnkDe9U/DistUpgradeControler.py", line 839, in run
    self.fullUpgrade()

  File "/tmp/tmpnkDe9U/DistUpgradeControler.py", line 817, in fullUpgrade
    if not self.askDistUpgrade():

  File "/tmp/tmpnkDe9U/DistUpgradeControler.py", line 515, in askDistUpgrade
    if not self._checkFreeSpace():

  File "/tmp/tmpnkDe9U/DistUpgradeControler.py", line 497, in _checkFreeSpace
    free_at_least = apt_pkg.SizeToStr(abs(fs_free[dir].free)+1)

TypeError: Only understand integers and floats