Ruler Measurement Units not working

I am trying to change the measurement units by clicking on the upper left hand corner of the rulers and nothing happens.
Is there some other way to change it?? I need to switch them to millimeters.

Panorama X does not currently have the ability to change measurement units in the ruler.

But Panorama X seems to adopt system settings. In System Preferences > International > Options you can at least switch between Metric / U.S. / UK units.

With my German standard settings, Panorama X rulers are set to cm.