IsNum and Only()

[cbaqir]

How do I use isnull in an only() statement?

=only({<[NOTE_TYPE] = {'TOP'},EXP_TYPE_CODE={'Operating'}, isnull(AU_NUM), isnull(MOR_TOWER)>}NOTE)

[vishsaggi]

May be this?

= only({<[NOTE_TYPE] = {'TOP'},EXP_TYPE_CODE={'Operating'}, AU_NUM = {"=Len(Trim(AU_NUM))= 0"}, MOR_TOWER = {"= Len(Trim(MOR_TOWER)) = 0"} >} NOTE)