SQL & PL/SQL

I am receiving following error, while executing a procedure kill_session

EXEC kill_session('USERNAME');

4329 267 TARIQ_CORE
ORA-00026: missing or invalid session ID

create or replace
procedure kill_session (usr_2_kill in varchar2)

as

vsid    number(6);
vserial number(6);

 begin

  for s_info in (select * from v$session where username=usr_2_kill) 
  
  loop
     vsid    := s_info.sid;
     vserial := s_info.serial#;

     dbms_output.put_line(vsid||'  '||vserial||'  '||usr_2_kill);
     
     execute immediate 'alter system kill session(:v1,:v2) immediate' using vsid,vserial;
          
  end loop;
   EXCEPTION
      WHEN OTHERS
      THEN
         dbms_output.put_line(SQLERRM);
   END;