Environment:SharePoint 2013, Workflow:Visual studio 2013 Declarative Workflow.

I’m trying to assigning task to user’s UPN (User Principle Name) instead of their EmailID but workflow getting suspended. Workflow can assign task to user’s email id successfully also workflow can successfully send email to EmailID as well as UPN also, but workflow get suspended while assign task to UPN with below error

System.ArgumentException: AssignedTo at Microsoft.Activities.Hosting.Runtime.Subroutine.SubroutineChild.Execute 

NOTE: If I manually assigning task & entering user’s Email OR UPN in “AssignedTo” field of task then user get resolved & task get assigned.

For your reference, Assume if user’s first name is AAA & last name is BBB then, email and UPN format in our environment is, Email: AAA.BBB@domain.com & UPN: ABBB@domain.com

Can someone know anything about this issue? Please help.