Disk space alert based on custom property

Hello everyone,

I'm trying to set up an alert for free space on physical volumes, based on available space and a custom property.  The custom property is set to whatever the threshold for that individual drive should be, as shown below

My alert is then set up like this:

I've tried several different configurations on this, such as setting the custom property to a floating point instead of integer.  I've also tried different entries for the custom property.  Right now it's set at 12582912, basically 12GB expressed in bytes, but I've also tried just 12.  No matter what combination I do, I can't get this alert to trigger.

Any suggestions or ideas for why I can't seem to get this to work?