Subtracting zero or subtracting from zero returns no result
I have a report in answers that show variances on the previous year. I have columns number of bookings this year, number of bookings last year and a variance column however when the figure for this year or last year is zero the variance returns nothing.i.e. if I had 10 bookings last year and none this year the variance is showing nothing when it should show -10. Any ideas ?