Can someone explain why this driver is not working? (very simple scale all 3 axis)

I am getting an odd effect when I try to create a driver to control all 3 scale axis of an object. The cube scales in a squash/stretch behavior when the same value is tied to all 3 axis.

Here is the sample scene. Scale is controlled by a single custom attribute for scale, but this is what is not working.

I have a workaround for this already (using 2 shapekeys instead of scale) but can anyone explain why this does not work? Also, is there a better way to lock all 3 axis together so they can be animated from a single curve?

Thanks.

Attachments

scaleDriver.blend (87.4 KB)

You have set a constraint to maintain its volume so you cannot uniformly scale all 3 axis. The scale increases but the object dimensions change to respect the original object volume

Dang it. I knew it had to be something simple. Had messed around with other constraints before using the custom property. Just forgot to delete that one.

Thanks!