Bug#503085:

[Tim Richardson]

To be clear, I fixed this problem by
1) logout so you see the login screen
2) click "options" (the button which lets you configure the login
screen; but on some themes, you go to Actions and find "Configure the
login manager"
3) enter root password
4) on the General tab, there is a button "edit commands" (bottom right)
5) edit the Command "Suspend command" to be /usr/sbin/pm-suspend

optional: Edit one of the custom commands to implement hibernate. The
path is /usr/sbin/pm-hibernate

6) Apply command changes