Help needed with derived field in ReportSmith

I have been trying to use Reportsmith to create a report with 12 amount
columns (one for every month of the year). I then set up a derived field
using the reports' SQL where I added all 12 fields together to give the
total of the 12 months. This works fine as long there are amounts in all
12 columns. As soon as I enter an amount in one month only (eg. January)
the derived Total SQL will not add it. The row in which the single amount
appears stays blank. Why is this ? Has the SQL a problem will zero amounts
in a field ?

Can anybody please help me ?

Thank You in advance

TIAN OLIVIER
SABC TV NEWS MANAGEMENT ACCOUNTANT