I have a callout pointing at a shape. I want the callout to show some of the properties of the shape. Say I want to point it at a shape that represents a server on a network diagram and I want it to show the server's IP Address

The custom formula for that field should look like this:
Microsoft's own documentation says "The "Prop" in the name is added automatically by Visio." but it isn't there. Prop doesn't exist!

Or at least that's what I think is happening. It'll say, "Error in formula." when I enter that custom formula. IntelliSense finds the CALLOUTTARGETREF but not the Prop part.

This is in Visio 2016 on Windows 10.