Form Validation Problems

I am checking to make sure that two drop down menus in a form have an option chosen before the form is submitted. I can accomplish this fine in IE, but Netscape still keeps submitting the form. Here is the code

<script language="javascript">
function chkData() {
if(document.productform.market.value == "") {
alert("You must choose a market.");
return false;
}
if(document.productform.prodtype.value == "") {
alert("You must choose a product type.");
return false;
}
else
return true;
}
</script>

onsubmit="return chkData()

can anyone tell me how to fix this so that netscape checks it also? Thanks