Formula which includes null values

Hi,

I am struggling to create a formula which caters for null values.

My formula contains IF( ZTAG2 > 0, ZTAG2, ZTAG1 ), but ZTAG2 or ZTAG1 can be null which is causing the result to be null. I need the result to be 0.

Is there a way that I can default ZTAG2 and ZTAG1 to have a zero value? They are both set up as numeric key figures.

Thanks