Hi @winfield,
As Tatyana already mentioned you will not be able to use Show Custom on columns in Value field well. You will have to return that particular value from your calculation, For Instance, if the value is NULL then you will pass back 0 which will then be used as part of sum/avg/ any other aggregation type you are intending to use.
In the case of avg, as Tatyana called out it will not be true average for example avg(0,100,200) is 100 where as the calculation should truly be avg(100,200) that comes out to 150.