Totalling Relationship Total Fields

Hello:

I am trying to create a Grand Total Field.   The goal is to total several relationship total fields (all existing in the Work Order Table. 

Playing with the formula, I can get one or two fields to total, but not always.
I am guessing it h as something to do with records having null values in those fields?

Just stuck after several hours of struggling (trying not to pester).

Thank you,

Dirk


[Total Labor Cost]+[Total Materials Cost]+[Total Materials Mark-up]+[Total Equipment Fee]+[Total Sub Contractor Price]+[Total Sub Contractor Mark-up]