"EmptyVarMessage" When using Email Notification

I'm tried to use Email Notification when there are "Alerts" Severity show up. 

I'm using command "$Message.MsgHost" in body email so that it will show Device IP Address/Hostname.

But when i try to test email notification, the "$Message.MsgHost" didn't show IP/Hostname of device, but only show "EmptyVarMessage"

Does any1 knows what should i do ?

My syslog Client was F0rtig4t3 Firewall.

Kindly Advise

Thank you.