If you are checking a cell for a zero value and the cell is blank, the test evaluates to true. For example, if you have the following formula in cell A1
and B1 is blank, the formula returns "zero" and not "blank" as expected.
If the range might contain a blank cell, you should use the ISBLANK function to test for a zero value, as in the following example:
Note that the above formula returns "zero" if there is a zero value in the cell, "blank" if the cell is blank, and "other" if anything else is in the cell.
You must always use the ISBLANK formula first before you test for a zero value. Otherwise you will always return a "true" for the zero value, and never get to the test for the ISBLANK formula.