CAS-6-Forgot username unable to sent user name in mail

I am new in CAS-6 and trying to send user name in mail, I created "forgot.password.gtemplat" file for email. In template I am trying to get username from attribute using this syntax "<%=principal.attributes.username %>", "<%=email %>" value of email is email address entered by user this returns proper value but value of <%=principal.attributes.username %> " is printing null, I didn't understand why, In console it says "None of the principal resolvers in the chain were able to produce a principal". For attribute resolution I am calling rest API.