รวมยอดเงินแล้วขึ้น NaN ทำไงดีครับ (คำนวนโดยจาวาคริป)

<script language="javascript">
function getanswer( )
{
var n1 =parseFloat(document.form1.bill_price1.value);
var n2 =parseFloat(document.form1.bill_price2.value);
var n3 =parseFloat(document.form1.bill_price3.value);
var n4 =parseFloat(document.form1.bill_price4.value);
var n5 =parseFloat(document.form1.bill_price5.value);
var n6 =parseFloat(document.form1.bill_price6.value);
var n7 =parseFloat(document.form1.bill_price7.value);
var n8 =parseFloat(document.form1.bill_price8.value);
var n9 =parseFloat(document.form1.bill_price9.value);
var n10 =parseFloat(document.form1.bill_price10.value);
var sum;
sum=n1+n2+n3+n4+n5+n6+n7+n8+n9+n10  ; 
document.form1.bill_total.value=sum;
}
</script>