Correct Parsing Of Strings For Numeric Values

I'm currently working on a web page that's acting much like a spreadsheet. There are fields that the user must fill in and then certain columns of fields add up to a sum at the bottom. I haven't been able to get it working quite right.

Here is the essence of the code
Code:
<form name="drillreport" method="post" action="process_report.php">
<table><tr>
<td> Field 1 </td>
<td> <input type="text" name="field1" id="field1" onBlur= calcTotal() /> </td>
</tr><tr><td> Field 2 </td>
<td> <input type="text" name="field2" id="field2" onBlur= calcTotal() /> </td>
</tr><tr><td> Field 3 </td>
<td> <input type="text" name="field3" id="field3" onBlur= calcTotal() /> </td>
</tr><tr><td> Total </td>
<td> <input type="text" name="tot_col" id="tot_col" onFocus= calcTotal() /> </td>
</tr></table>
...
It is more than that of course, but you get the idea.

The function I wrote is the following:
Code:
<script type="text/javascript">
function calcTotal() {
var returned_val=0;
returned_val += parseFloat(document.drillreport.field1.value);
returned_val += parseFloat(document.drillreport.field2.value);
returned_val += parseFloat(document.drillreport.field3.value);
document.drillreport.tot_col.value = returned_val;
}
</script>

I use the onBlur for each of the "component fields" so that if the user changes one of the fields, it updates the total. However, I can't get this to return values properly when just field1 is entered ( NaN ) comes up. As soon as I enter the second column the total appears fine. Why wont this work if you just enter the first field?