Map file to convert to LowerCase

Hi Team, 

I want to convert session start and session end user name to lower case using map file. My map file looks like this. But it is not changing the case. Can you please suggest how to rectify this issue.

event.deviceVendor,event.deviceProduct,event.name,set.expr(sourceUserName).event.sourceUserName
Pulse Connect Secure,Pulse Secure,Conn Closed,__toLowerCase(sourceUserName)
Pulse Connect Secure,Pulse Secure,Session Start,__toLowerCase(sourceUserName)