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?