I am using token calculator to increase a value (sum counter) for a collection of documents with a "sales tax amount" as one of the fields, to get a Total sales tax for the total collection sales tax value.  This works fine as long as there is a number, even zero, in the sales tax amount field.  However, if the field is simply blank, the calculator yields a blank.  I would like to to infer a zero for a blank field.  For example: If I have 16 items in the collection and one has a blank field, my Total comes up blank.  Any ideas???