Description of problem:
I'm using Fedora 18 with MATE, and I've just clicked at System > Administration > Users and Groups.

Version-Release number of selected component:
mate-polkit-1.5.0-1.fc18

Additional info:
backtrace_rating: 4
cmdline:        /usr/libexec/polkit-mate-authentication-agent-1
crash_function: g_type_check_instance_cast
executable:     /usr/libexec/polkit-mate-authentication-agent-1
kernel:         3.8.4-202.fc18.x86_64
uid:            1000
ureports_counter: 1
var_log_messages: Mar 31 15:14:59 jcagnoni abrt[3556]: Saved core dump of pid 1284 (/usr/libexec/polkit-mate-authentication-agent-1) to /var/tmp/abrt/ccpp-2013-03-31-15:14:59-1284 (22409216 bytes)
xsession_errors: 

Truncated backtrace:
Thread no. 1 (9 frames)
 #0 g_type_check_instance_cast at gtype.c:3992
 #1 polkit_mate_authentication_dialog_constructed at polkitmateauthenticationdialog.c:551
 #3 g_object_new_valist at gobject.c:1835
 #5 polkit_mate_authentication_dialog_new at polkitmateauthenticationdialog.c:857
 #6 polkit_mate_authenticator_new at polkitmateauthenticator.c:252
 #7 polkit_mate_listener_initiate_authentication at polkitmatelistener.c:200
 #8 auth_agent_handle_begin_authentication at polkitagentlistener.c:672
 #9 auth_agent_handle_method_call at polkitagentlistener.c:294
 #10 call_in_idle_cb at gdbusconnection.c:4737