NullString Property
From Microsoft Excel Visual Basic
Returns or sets the string displayed in cells that contain null values when the DisplayNullString property is True. The default value is an empty string (""). Read/write String.
Example
This example causes the PivotTable report to display "NA" in cells that contain null values.
With Worksheets(1).PivotTables("Pivot1")
.NullString = "NA"
.DisplayNullString = True
End With