We might need more information to help you to work this out, but if what you are trying to do is get the sum of two Columns from a Database Table, then two obvious things to look for are:
Do both of the Columns contain Numeric type data?
Does either (or both) of the Columns contain a Null Value?
Nz() is a new and improved function that replaced the A97 NullToZero() function. The added improvement to this is that the earlier function only changed a null value to zero, whereas Nz() allows you to use any replacement value you want.
For Example
Yesterday = Nz([AnyDate],Date()-1)
Therefore if their is no value in the anydate field it will replace it with yesterday's date.
Likewise lets say you are doing a calculation and one side of the equasion = 0 you could get a division by zero error, so if you then used the Nz() like Nz([AnyValue],1) would eliminate the error as timesing anything by one will give you the same figure.
Thanks David. That's the best most concise explanation I've had in a while. I've never used NZ before but, now I know that it exists I can see where it could have solved a few problems for me.