Help with SWQL Alert

Hi,

I wanted to use the existing "Out of the Box" 'Alert me when memory has less then 60 Days of Capacity" but I want to only alert on a subset of my servers.  I ideally want to use DeviceType = "Domain Controllers"  so I added in Where node.customproperties.devicetype = "Domain Controllers" but when I try to validate it the queries fails.

Please can anyone shed some light for me?

Regards

Patrick