backintime (root) won't start via menu, same command works on cmd line

I'm using linux mint 17.3. I had an older version of backintime and couldn't display hidden files to restore them, so I installed 1.1.12. There's a menu item for backintime and backintime (root). The former works, but the latter doesn't launch. But if I copy the command from the menu item and paste it into a terminal, it runs. Baffling. Any idea how to make the menu item work properly? I've tried creating a new one and the new one also doesn't work. The command in the menu (and the one that works on the cmd line) is "pkexec backintime-qt4". Thanks in advance!