Sum of other math fields

I'm trying to use a math field as a sum of other math fields. For example, on a quote form there are multiple math fields for different totals (Not all are used though) and I am trying to use a math field for Grand Total and using the expression [field_total1] + [field_total2] + [field_total3] etc. If one of the totals is not used it does not work, is there a way to do a sum of or default a math field to 0 maybe?

Any help appreciated.