Bit of a strange one this.....

I have been sent a workbook which tracks staff absences, holidays etc.

There is a fomula which calculates the total number of days off, holidays
etc. by summing up a specific cell value from each month tab of the workbook.
This works fine but if a user changes one of these cell values (from blank
to 1 for example) but then decides to delete the number entered (leaving the
cell blank again) the calculated "Sum" cell on the summary sheet then
displays a "#Value" error. Try as I might I cannot replicate this error in
Excel 2007 and don't understand why I am getting this error in Excel 2003??
Why not just use 2007 then? Because my boss's PC is running 2003 and it's
for him!

The strange thing is that my collegue who created the sheet, used Excel
2007. I then opened the sheet (also in Excel 2007) and did not get the
#Value error when doing the above. But when my collegue opens the sheet in
Excel 2003 he gets the #Value error???

My question is why and how do I stop the #Value error?