This is not what I am getting when I do the same thing. Would you provide your data? The average of 0 is 0. However your .jpg suggests that you are using the AVG( Contract Room ADR) but two fields you are using say Contract Room Revenue and Contract Rooms Sold. I am unsure what the formula you are using is but if Contract Room Revenue is dividing one value by the other and then you take the average of the quotient you will get a null. This is because anything divided by 0 is null, as it cannot exist. (Maths!)
If you must override this logic and use 0 instead please use the following calculation.
What this does is forces the cell to be addressed. When it is addressed and found to be null (if my above guesses are correct, it will be). It will then force the calculation to be 0 instead of null.
Carl J. Slifer