If your task has been entered automatically you will not have NA in either Duration or Number fields. Number will be 0 if nothing has been entered and duration will be 0 Days.
As such you can put in an error catch for this:
iif([duration2]=0 OR [number1]=0),0, ENTER your formula)

If you have entered a task as manual you might have a blank or unusual entry but you can trap this as well with Task Type = Manual

HTH

Miles