It's a little difficult to determine exactly where you're trying to use this. Is it in a pivot table or standard table? If so then I assume you've got something like
Sum(Value) as the measure and you're saying if that's null then make it zero?
If that's what your'e trying to do then try this.
If(IsNull(Sum(Value)), 0, Sum(Value))
If that's not what you're trying to do then please post an example.