Forms :: Calling Reports 9i - Argument Can't Be Null

I am calling report9i from forms9i. i am getting following error

invalid report id

when i call a report from form in when button pressed trigger. And also giving the following error

argument(1) can't be null.

v_rep varchar2(100);
v_rep := run_report_object('report5',pl_id);

or

rep_object report_object;
v_rep varchar2(100);

rep_object := find_report('report5');
v_rep := run_report_object(rep_object,pl_id);

where pl_id is the parameter list.