How to set the display message for a validator on a cloned issue

[lorraine wildman]

Rule: Clone a defect "customfield" cannot be left blank while creating a defect.  The field is mandatory. Present a message and block clone process if field is empty. 

Action:  On the Create Issue event, attempted to add a validator ..

string ERRORMSG = "ERROR";

string[] mycf = "customfield_11847";
if(!isNotNull("mycf"))

{ lfGlobalMessage("Applicaion List must have a value", "ERROR"); return false, mycf, "ERROR"; }

—

 On execution:  I am getting the correct popup, but the message is not correct. Getting the system message instead of custom message.

"admin.errors.exception com.atlassian.jira.exception.CreateException: Error occurred while creating issue through workflow: "

How to change the system ;message to my custom message?