a null value wanted to be added

[Rasha]

i design a report in design view and i insert text box that will add two fields together


i found a problem that if one of these field have a null value the returned value is empty
it can't add a number to a null value
this is the formula which i wrote in the text box
=[Medical Gases_CO2 - Carbon Dioxide]+[CO2_Items_Room]
the problem is usually be in the first part of the formula
any body can help me

[Bob Fitz]

Try:
=Nz([Medical Gases_CO2 - Carbon Dioxide],0)+Nz([CO2_Items_Room],0)