DIVIDE function when dealing with 2 zeroes

Hi all !

I have a discrepancy measure whose value is : 

Discrep :=
1
    - DIVIDE ( ABS ( A1 - B1 ), A1, 1 )

So it should return 0 whenever A1 is equal to 0. But here, when I come to check the results, I see that it only makes sense when one of A1 or B1 has a value. When both are null, the divide function seems to return 0 instead of the 1 parameter I passed to it.

Is there some kind of nuance where it doesn't consider 0 and null to be the same and so divide(null,null,1) = null ? If not do you have an idea why it does that ?

Thanks in advance !