IsBlank And Inline Javascript

I have a function that is used to take the contents of various text
boxes, sum them, and write the results to another text box. I have
this function set-up to handle the constiuent text boxes' onBlur
events, and it works great. One of the things it does is it uses the
isBlank method of the string containing the contents of each text box
before it tries to do math on it:

function checkTotal(clientID,id) {
..
..
..
e = document.getElementById(clientID);
if (e) {
strValue=e.value.toString();
alert(strValue);
if (!strValue.isBlank())
..
..
..

I want this script to run when the page loads. This code is generated
deep in an ASP.Net user control, and I can't come-up with a good way to
execute this code in the page's body's onLoad. So I am writing the
following as in-line JavaScript at the bottom of the FORM:

<script language="JavaScript">
checkTotal('txtBudgetLaborHours','BudgetLaborHours ');
</script>

The function executes fine up until it gets to the isBlank method.
I've used alert's to verify that the text box in fact does have the
proper value when this method is called, but when the script goes to
execute the isBlank method I'm getting an "object does not support this
property or method" error.

Has anyone seen anything like this before? Is there a reason the
isBlank function isn't available at this point in the page's life?