Problema con el Centro de Software

Bueno pues soy muy nuevo en esto y desde esta mañana no he podido ingresar al centro de software, desde la terminar me muestra lo siguiente:

zteve@zteve-Aspire-4736:~$ software-center
2013-01-25 13:43:19,072 - softwarecenter.ui.gtk3.app - INFO - setting up proxy 'None'
2013-01-25 13:43:19,082 - softwarecenter.db.database - INFO - open() database: path=None use_axi=True use_agent=True
2013-01-25 13:43:19,744 - softwarecenter.region - WARNING - failed to use geoclue: 'org.freedesktop.DBus.Error.ServiceUnknown: The name org.freedesktop.Geoclue.Master was not provided by any .service files'
2013-01-25 13:43:20,122 - softwarecenter.backend.reviews - WARNING - Could not get usefulness from server, no username in config file
2013-01-25 13:43:20,130 - softwarecenter.fixme - WARNING - logs to the root logger: '('/usr/lib/python2.7/dist-packages/gi/importer.py', 51, 'find_module')'
2013-01-25 13:43:20,130 - root - ERROR - Could not find any typelib for LaunchpadIntegration
2013-01-25 13:43:20,300 - softwarecenter.ui.gtk3.app - INFO - show_available_packages: search_text is '', app is None.
2013-01-25 13:43:20,679 - softwarecenter.db.pkginfo_impl.aptcache - INFO - aptcache.open()
Traceback (most recent call last):
File "/usr/share/software-center/softwarecenter/db/pkginfo_impl/aptcache.py", line 257, in open
self._cache = apt.Cache(progress)
File "/usr/lib/python2.7/dist-packages/apt/cache.py", line 102, in __init__
self.open(progress)
File "/usr/lib/python2.7/dist-packages/apt/cache.py", line 149, in open
self._list.read_main_list()
SystemError: E:Línea 56 mal formada en la lista de fuentes /etc/apt/sources.list (análisis de URI)
2013-01-25 13:43:23,705 - softwarecenter.fixme - WARNING - logs to the root logger: '('/usr/lib/python2.7/dist-packages/gi/importer.py', 51, 'find_module')'
2013-01-25 13:43:23,705 - root - ERROR - Could not find any typelib for Gst
2013-01-25 13:43:24,052 - softwarecenter.db.enquire - ERROR - _get_estimate_nr_apps_and_nr_pkgs failed
Traceback (most recent call last):
File "/usr/share/software-center/softwarecenter/db/enquire.py", line 115, in _get_estimate_nr_apps_and_nr_pkgs
tmp_matches = enquire.get_mset(0, len(self.db), None, xfilter)
File "/usr/share/software-center/softwarecenter/db/appfilter.py", line 89, in __call__
if (not pkgname in self.cache and
File "/usr/share/software-center/softwarecenter/db/pkginfo_impl/aptcache.py", line 277, in __contains__
return self._cache.__contains__(k)
AttributeError: 'NoneType' object has no attribute '__contains__'
Traceback (most recent call last):
File "/usr/bin/software-center", line 182, in
app.run(args)
File "/usr/share/software-center/softwarecenter/ui/gtk3/app.py", line 1387, in run
self.show_available_packages(args)
File "/usr/share/software-center/softwarecenter/ui/gtk3/app.py", line 1325, in show_available_packages
self.view_manager.set_active_view(ViewPages.AVAILABLE)
File "/usr/share/software-center/softwarecenter/ui/gtk3/session/viewmanager.py", line 151, in set_active_view
view_widget.init_view()
File "/usr/share/software-center/softwarecenter/ui/gtk3/panes/availablepane.py", line 173, in init_view
self.cache, self.db, self.icons, self.apps_filter)
File "/usr/share/software-center/softwarecenter/ui/gtk3/views/lobbyview.py", line 80, in __init__
self.build()
File "/usr/share/software-center/softwarecenter/ui/gtk3/views/lobbyview.py", line 324, in build
self._build_homepage_view()
File "/usr/share/software-center/softwarecenter/ui/gtk3/views/lobbyview.py", line 119, in _build_homepage_view
self._append_whats_new()
File "/usr/share/software-center/softwarecenter/ui/gtk3/views/lobbyview.py", line 253, in _append_whats_new
whats_new_cat = self._update_whats_new_content()
File "/usr/share/software-center/softwarecenter/ui/gtk3/views/lobbyview.py", line 238, in _update_whats_new_content
docs = whats_new_cat.get_documents(self.db)
File "/usr/share/software-center/softwarecenter/db/categories.py", line 131, in get_documents
nonblocking_load=False)
File "/usr/share/software-center/softwarecenter/db/enquire.py", line 330, in set_query
self._blocking_perform_search()
File "/usr/share/software-center/softwarecenter/db/enquire.py", line 225, in _blocking_perform_search
matches = enquire.get_mset(0, self.limit, None, xfilter)
File "/usr/share/software-center/softwarecenter/db/appfilter.py", line 89, in __call__
if (not pkgname in self.cache and
File "/usr/share/software-center/softwarecenter/db/pkginfo_impl/aptcache.py", line 277, in __contains__
return self._cache.__contains__(k)
AttributeError: 'NoneType' object has no attribute '__contains__'
zteve@zteve-Aspire-4736:~$

Espero y puedan ayudarme gracias por su atención, saludos.