Simple Variable/Driver Issue

Mik Lip Sync.blend (4.55 MB)

Hi all.

I’m wondering how to set the variable path of the driver to the custom property “prop” at the bottom right corner, but it seems to be impossible.
The end goal is that my character’s mouth should be moving up and down, however, it is for some reason impossible for the path to be recognized within the variable.

Help is much appreciated! I’m wondering if this issue is too complex however…

Thank you for reading, and if you can solve it please tell me what I did wrong.