ORA-04079:INVALID INTEGER SPECIFICATION ....please resolve this : create or replace trigger fineCalc AFTER UPDATE ON book_issue for each row
when ((new.date_of_return-old.date_of_issue)>7) 
declare 
rcpt_no  number;
s_id  char(10);
begin
if :old.card_id in (select card_id from STUDENT_BOOK_ISSUE)
then
select max(receipt_no) into rcpt_no from fine;
select student_id into s_id from STUDENT_BOOK_ISSUE sbi where sbi.card_id=:old.card_id;

    insert into fine values( rcpt_no+1,((:NEW.date_of_return-:OLD.date_of_issue-7)*5),s_id);
end if;
end;