set_field packaged procedure

Hi all,

I am attempting to use the 'set_field' packaged procedure in SQL*Forms 3.0 with Oracle V6, but don't understand how it works.

What I want to be able to do is alter the characteristic of a field depending upon the value of some other field, something along the lines of :-

ON-VALIDATE-FIELD Trigger on :b1.field1

if :b1.field1 = 'T' then
 set_field(:b1.field2,ENTERABLE,ATTR_ON); else
 set_field(:b1.field2,ENTERABLE,ATTR_OFF); end if;

This gives the following error :-
FRM-40738: PL/SQL error -- null argument to procedure not allowed.

Can somebody explain what the problem is and how set_field should be used ?

Thanks in advance ..
Tony

-- 
----------------------------------------------------------------------------
Tony Desborough                                                 SRL Data Ltd
Software Programmer                                             1, Perren St
Email: tony_at_bilpin.co.uk                                     LONDON, NW5 3ED