Why isn't that null

Moin,

I have al small problem with ISNULL in a trigger
function. My definition looks like:

IF NEW.vkp ISNULL THEN
   select t1.las into i_kd
   from auftrag t1, aufpos t2 where
   t2.auftrag = t1.id;
   select get_vkp(i_kd, NEW.artikel) into f_vkp;
   NEW.vkp:=f_vkp;
END IF;

The following SQL query didnt cause this if statement:

insert into aufpos(auftrag, artikel, best_menge, vkp,
lieferwoche, cuser, ctime, uuser, utime) values
('175','8501900','12','','3500','matthias',
now (), 'matthias',now ())

If I change the function statement as follows:

IF NEW.vkp = 0 THEN
   ...
END IF;

it works but '0' is a leagal value but not '' (empty). The
function should only be called if NEW.vkp ist empty.

Many thanks
Matthias