Host key does not exist error when adding Eventlog trigger

[scottmace]

When I try to add this trigger:
{Template_Windows Events:eventlog[Application,,"Warning|Error"].logseverity(0)}=1

I get:
Host key does not exist. Check expression part starting from ' :eventlog[Application,,"Warning|Error"].logseverity(0)}=1 '

Item key looks like this:
eventlog[Application,,"Warning|Error"]

And we have six hosts using this template. Any thoughts on how to troubleshoot this?