=SUM(ABOVE) give incorrect result when there is only one row above

I am using a Word Template for preparing invoices for quite some time. This
Template is serving my purpose better than an Excel Template because of
better word processing capabilities of Word along with basic calculations
support in tables.

However, in my Template a formula [=SUM(ABOVE)] in a table give obviously
incorrect result when there is only one row above. I can attach a copy of
this Template if it is allowed by this Discussion Group, or send it by email.
Meanwhile I try to explain the situation below:

This invoice would often have three or more Parts, e.g. Part-A, Part-B, …
Below each Part there is a row for Sub Total Part-X, and in this the Sub
Total is calculated with =SUM(ABOVE). But in cases when there is only one row
in a given Part, the Sub Total Part-X [i.e. =SUM(ABOVE)] give obviously
incorrect result. The result is calculated correctly as soon as I insert one
or more rows in this Part.

Luckily, the final Total is not affected by such incorrect =SUM(ABOVE)
formula result because it use a formula =SUM(F:F). This work because in all
the row of Sub Total Part-X, the Sub Total is only visually displayed below
column F but due to merged cells it is not actually in column F. =SUM(ABOVE)
sums all consecutive rows visually above it but =SUM(F:F) sum all numbers in
column F.

So I have to manually type in the Sub Total Part-X where there is only one
row in a given Part-X. Am I doing something wrong or is there a work around?
A can attach a copy of this Template if it is allowed.

N.K.Saini