MySQL Forums :: MySQL Monitor :: unmappable variable when attempting to schedule a new rule

I am attempting to create and schedule a new rule for Enterprise Monitor but am receiving a java error.

Steps:
1) MySQL Monitor Agent - create a custom query in custom.xml. Restart the Agent.
2) MySQL Enterprise Monitor: Manage Rules->Create Rule (and Advisor).

No errors so far.

However when I attempt to add the new rule to the schedule, I get:
UU59 There was a problem scheduling "new query" against "server"

Looking at ALL logs, I see:

java.lang.RuntimeException: Unmappable variable %my_var% for "data item" and instance local on server "server"
at com.mysql.etools.monitor.bo.InstanceFinder.findInstance(InstanceFinder.java:103)

I haven't been able to figure out why this variable is unmappable - I've checked the spelling, database, etc and everything looks okay to me. Has anyone seen this problem? Any idea what might be causing it?