I can't change my graphics through nvidia-settings

when I change my graphics in 'NVIDIA X Server Settings'-'PRIME Profiles', it pop a window, and my terminal display some infomations:

ERROR: Traceback (most recent call last):
       File "/usr/lib/python3/dist-packages/dbus/bus.py", line 175, in
       activate_name_owner
       return self.get_name_owner(bus_name)
       File "/usr/lib/python3/dist-packages/dbus/bus.py", line 361, in
       get_name_owner
       's', (bus_name,), **keywords)
       File "/usr/lib/python3/dist-packages/dbus/connection.py", line 651, in
       call_blocking
       message, timeout)
       dbus.exceptions.DBusException:
       org.freedesktop.DBus.Error.NameHasNoOwner: Could not get owner of name
       'com.ubuntu.ScreenResolution.Mechanism': no such name
       During handling of the above exception, another exception occurred:
       Traceback (most recent call last):
       File "/usr/share/screen-resolution-extra/nvidia-prime.py", line 55, in
       <module>
       conf = get_prime_service()
       File "/usr/share/screen-resolution-extra/nvidia-prime.py", line 34, in
       get_prime_service
       service_object = dbus.SystemBus().get_object(SERVICE_NAME, OBJECT_PATH)
       File "/usr/lib/python3/dist-packages/dbus/bus.py", line 241, in
       get_object
       follow_name_owner_changes=follow_name_owner_changes)
       File "/usr/lib/python3/dist-packages/dbus/proxies.py", line 248, in
       __init__
       self._named_service = conn.activate_name_owner(bus_name)
       File "/usr/lib/python3/dist-packages/dbus/bus.py", line 180, in
       activate_name_owner
       self.start_service_by_name(bus_name)
       File "/usr/lib/python3/dist-packages/dbus/bus.py", line 278, in
       start_service_by_name
       'su', (bus_name, flags)))
       File "/usr/lib/python3/dist-packages/dbus/connection.py", line 651, in
       call_blocking
       message, timeout)
       dbus.exceptions.DBusException:
       org.freedesktop.DBus.Error.Spawn.ChildExited: Launch helper exited with
       unknown return code 1

my nvidia-prime version is 0.5.5.
my nvidia version is 331.20
my system is ubuntu 14.04 daily-build, 3.13.0-8-generic .
my separete graphics is GT 630M.

What's wrong with it? And what should I do ?
Thanks.