APEX

hi,

i have created a form with report Page .When i click on Edit ICON And Go to Edit Page ,On that page i am using an Java Script

<script>
function servicetaxItems(){
function getVal(item){
if($x(item).value != "")
return parseFloat($x(item).value);
else
return 0;
}
$x('P37_SERVICE_TAX_AMT').value =
(getVal('P37_SERVICE_TAX') * getVal('P37_LAB_AMT')/100);

}
</script>


I have put This on HTML Form Element Attributes onBlur="javascript:servicetaxItems()"

Now Problem is When I changes in P37_LAB_AMT and my Cursor Come in P37_SERVICE_TAX_AMT Item Then It's Shows NAN .
i don't know why it is show NAN ..
I want Calculated Value In P37_SERVICE_TAX_AMT


How Can i Remove This Error ..

Thanks