Here's our Microsoft Office Tip for.. May 19, 2003
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Watch Out For Nulls
In general, you'll want to avoid using or referring to a Null value in your expressions. That's because a Null in any mathematical expression causes the entire expression to evaluate to Null. For instance, the simple expression

2 + Null

will return Null, whereas the expression

0 + 2

will return 2.

This behavior is true whether you're working with a simple expression or VBA code.