IE issue with custom jQuery extender

My code works with the newest versions of Chrome and Firefox, but doesn't work with IE9,( I don't have earlier versions to test yet).

Here is my jQuery extender

(function ($) {
    $.fn.myModal = function() {

        var offsetx = this.width() / 2;
        var offsety = this.height() / 2;
        var bg = $("<div id='modal-overlay'></div>");
        $("body").append(bg);
        //etc.....                  
    }
}) ( jQuery );

I used a try/catch block to开发者_开发问答 determine which lines of code is throwing an error and it appears that any reference to this is the problem. Even if I wrap this with $(this) it still doesn't work.

Btw I'm calling this extender like so:

$(function ()
{
    $("#login").click(function ()
    {
        $("#login-modal").myModal();
    });

});

For what it's worth all of this code is in an external file which gets called in the head of the document.