Reports & Discoverer :: Error Messages Handling In Report

I am generating a report with parameter form.

I do not want users to leave a parameter value null . so I applied after parameter form trigger

function AfterPForm return boolean is
begin
IF ( :my_no IS NULL ) THEN
srw.message(101, 'Please enter proper No.');
return (FALSE);
END IF;
return (TRUE);
end;

it is working fine, it displays proper message but after my message, a default message is displayed

REP-0771: After Form Trigger Failed.

I want to avoid this message. if i comment out return(false) then it displays message and runs the report. I want to avoid default oracle message and stop running report also.