IIF with IsNull

[canett]

I have a good IIF Function based on the availablity of a field, which sometimes might be 0 and sometimes there is nothing in it.My function works, if the filed cartons is either 0 or > 0. However, my function does not work if the field cartons is blank, i.e. there is no 0 in it.
How an I improve my Iif function so to include both three cases : when it is Null, hen it is 0 and when it >0 ?

NetPrice: IIf([cartons]>0;[ddp]*[cartons]*[size]*[pack];IIf([cartons]=0;[ddp]*[size]*[quantity];0))

 

[JerryKlmns]

NetPrice: [ddp]*[size]*IIf(nz([cartons],0)>0;[cartons]*[pack];[quantity])