Extender fails after partial postback in IE7 only

I have a simple extender which sets focus to a textbox when the page loads. In IE after a partial postback this control doesn't reset the focus. It does work on page load however and it works every time in FireFox. Any suggestions? Below is the JS for the extender:

Type.registerNamespace('AjaxFocus');

AjaxFocus.AjaxFocusBehavior = function(element)
{
AjaxFocus.AjaxFocusBehavior.initializeBase(this, [element]);
}

AjaxFocus.AjaxFocusBehavior.prototype =
{
initialize : function()
{
AjaxFocus.AjaxFocusBehavior.callBaseMethod(this, 'initialize');

var e = this.get_element();

if (e != null)
e.focus();
},

dispose : function()
{
AjaxFocus.AjaxFocusBehavior.callBaseMethod(this, 'dispose');
}
}

AjaxFocus.AjaxFocusBehavior.registerClass('AjaxFocus.AjaxFocusBehavior', AjaxControlToolkit.BehaviorBase);

Is the Extender inside of the update panel?

Extended class and debug="false"

Does anybody know why the initalizeBase() function fails to extend the current class with the base-class's methods when debug is set to false?

It seems that when in debug mode you are allowed to do this use the parent class' initializeBase function in the constructor. This causes an error with debug=false.

By using the subclass's own initalizeBase, this all works like a charm.