I often use sumif formula in spreadsheets, the criteria usually is a
cell reference so the user can decide what they want to add up.

I started to get some results for sumifs when the criteria cell was
blank. On investigation I found that there were some zero values in
the spreadsheet and that the formula was counting them.

I thought a blank cell was a null value and not a zero value. I can
duplicate this on different excel installations on different computers
though the versions are all 2002.