JQuery UI - "dialog is not a function" Hunh?

I get the error message shown below when I click the button: ($'#billing_button_addTime')

billingController.dialogAddTime.dialog is not a functio开发者_开发技巧n

billingController is created in $(document).ready.

function BillingController() {
    }
BillingController.prototype.dialogAddTime = $(document.createElement('div'));

BillingController.prototype.loadBillingContent = function () {
        //Load the main content    
        $("#content_area").load('/Content/HTML/billing.html', null, mainController.attachScrollBarsToCPOTable);

        //Pre-Load the Add Time Dialog
        $(billingController.dialogAddTime).load('/Content/HTML/billing_dialog_addTime.html', null, billingController.bindButtonAddTimeToHandler );
    }

BillingController.prototype.bindButtonAddTimeToHandler = function () {
    $('#billing_button_addTime').bind('click', billingController.buttonHanderAddTime);
}

BillingController.prototype.buttonHanderAddTime = function () {
    billingController.dialogAddTime.dialog({ modal: true });
}