Application Express :: Checkbox On Click Action

I am working in apex 4.2.1.

I'm having an odd problem with a checkbox on a tabular form. I created the checkbox by setting hte display as to "Simple Checkbox". The value of the underlying column sets to "Y" wehn checked and "N" when unchecked. This is what I want. The next thing I was trying to accomplish is to call a javascript function when the checkbox is clicked. So I put onclick="calculateTotal(this);" in the element attributes field. The code that it is calling is listed below. When I run the app and check the box I can seee I am entering the calculate Total function. Each of the alert functions is displaying what I would expect until I get to alerts 'IAH5' and 'IAH6'. For each of those I get an "undefined" in the alert.

So to see if this had something to do with the checkbox and added an onchange="calculateTotal(this);" to another field in my tabular form and ran it. This time I did not click on the checkbox but went into the field that I put the onchange on. When I changed this field and navigated away from it the function worked as I would expect. It displayed the appropriate values for each of the alerts and updated the fields on the screen as designed.

<script type="text/javascript">
function calculateTotal(pThis)
{
alert('IAH1');
var vRow = pThis.id.substr(pThis.id.indexOf('_')+1);
alert('IAH2');
[code].......