calculation from query

I've a query where I keep some fields:

btw
excl
incl
total

I use this query as subform, now here 's my problem

I've got a textfield 'total' where I want to calculate the total from
the query for the loaded form.

I've used the expressionbuilder to perform this:

=Sum([subfrm FaktuurTest].Form!Inclusief)

When I see the form in 'formview' I get the following message:

#Error

Someone got any id?