No authentication agent found.

Trying to run gparted results in this error message:

$ gparted
Error executing command as another user: No authentication agent found.

I've finished setting up polkit first, and that works fine:

$ pkttyagent -p $(echo $$) | pkexec whoami
==== AUTHENTICATING FOR org.freedesktop.policykit.exec ====
Authentication is needed to run `/usr/bin/whoami' as the super user
Authenticating as: hugo
Password:
==== AUTHENTICATION COMPLETE ====
root

The agent is obviously there and works fine, since it prompts me for a password and runs whoami as root.

Given that an agent is present and working, I'm assuming the failure is on gparted's side.

I don't think the problem is polkit either, I've configure it to be rather straightforward:

polkit.addRule(function(action, subject) {
    if (subject.isInGroup("wheel")) {
        return polkit.Result.AUTH_SELF;
    }
});