Main windows fails with "SystemError" if malformed line in sources.list

update-manager fails to show its dialog when /etc/apt/sources.list contains malformed line:

$ update-manager
/usr/lib/python2.4/site-packages/apt/__init__.py:17: FutureWarning: apt API not stable yet
  warnings.warn("apt API not stable yet", FutureWarning)
Introspect error: The name org.freedesktop.UpdateManager was not provided by any .service files
no listening object (The name org.freedesktop.UpdateManager was not provided by any .service files)
Traceback (most recent call last):
  File "/usr/bin/update-manager", line 89, in ?
    app.main(options)
  File "/usr/lib/python2.4/site-packages/UpdateManager/UpdateManager.py", line 948, in main
    self.fillstore()
  File "/usr/lib/python2.4/site-packages/UpdateManager/UpdateManager.py", line 800, in fillstore
    self.initCache()
  File "/usr/lib/python2.4/site-packages/UpdateManager/UpdateManager.py", line 886, in initCache
    self.cache = MyCache(progress)
  File "/usr/lib/python2.4/site-packages/UpdateManager/UpdateManager.py", line 87, in __init__
    apt.Cache.__init__(self, progress)
  File "/usr/lib/python2.4/site-packages/apt/cache.py", line 36, in __init__
    self.open(progress)
  File "/usr/lib/python2.4/site-packages/apt/cache.py", line 53, in open
    self._cache = apt_pkg.GetCache(progress)
SystemError: E:Malformed line 53 in source list /etc/apt/sources.list (dist parse), E:The list of sources could not be read.

aptitude says
$ sudo aptitude update
Password:
E: Malformed line 53 in source list /etc/apt/sources.list (dist parse)
E: Malformed line 53 in source list /etc/apt/sources.list (dist parse)
E: The list of sources could not be read.