acrobat defaults to 0.00 in sum field

Hi - I'm calculating a SUM from 2 fields - but acrobat is insisting on writing 0.00 as default. I don't see any way to change that behaviour, without deselecting the "sum" radio button and using custom script. Or did I miss something?. Is there an easy script that does both the sum command and a null command?
Maybe something that would add to this?
if(event.value == 0) event.value = "";