#Error on Subform

[phuser]

Main Form contains Unit Information and Totals in text boxes brought down
from the Subform. The subform filters by UnitID and I am trying to calculate
total spent on repairs for this unit, I created a text box on the subform
=Sum([Parts]) and =Sum[(Labor]) and =Sum([Total]) but I still get the #Error
in all 3 columns, some of the invoices (rows) do not contain Labor or Parts
but each row has a total. What else can I try to get the calculations?

 

[phuser]

Ok maybe a little background will help, now that I've created a query to try
and total things up I see where the problem lies,
I dont know how to fix it but I sure hope I can describe it well enough so
one of you guys can help.
I took the forms from the MS template of Service Call Management Data Base.
I followed the same format, creating a seperate table for Parts Details &
another for Labor Details these tables are joined to the Workorder Table and
flow seems good, till I try to get totals when an invoice contains 1 or the
other but not both, the cell in the query for either/or is blank, this query
is the one used to make my summary screen, so I get #Errors in every column.
If the query were to produce 0's in the blank cells the numbers could then
be added up right? Tried the nz in the sum of the subform and still got the
#Error.