Question : #Error when sub form has no value

Hi EE
I have a field on a form with a sum as the control:
=(([qryCountSalesCallStat subform].Form!Count_No_Answer)+([qryCountSalesCallStat subform].Form![Line Busy])+([qryCountSalesCallStat subform].Form![Answer Phone]))

This works fine when the subform has a value however when you create a new record this field always displays #Error message as its trying to add up a value at this stage doesnt exist.
I need this to say 0 instead of #Error.

I have tried using an iif formula but cant get it to work

How can i re-write the formula so if the subreport it refers to has not yet a value it returns a 0 and not a #Error

Answer : #Error when sub form has no value

txtBox1= DMax ("field","table") + 1