HTML5 excanvas "Object doesn't support this property or method"

I'm building a little HTML5 practice page at www.MyDoodleDot.com and was having problems getting "excanvas", the shim for using the HTML5 canvas element in older IE browsers. In my HTML I included the conditional running of the excanvas shim with this:

<!--[if lt IE 9]><script type="text/javascript" src="excanvas.compiled.js"></script><![endif]-->

But when IE got to the second line below, it would die with "Object doesn't support this property or method".

    var canvas = document.getElementById("mycanvas");
    var context = canvas.getContext("2d");

The problem was that excanvas.js had not been fully loaded, so when IE hit that line, the canvas object didn't have a "getContext()" method. This happened because I was using this to start my javascript,

$(document).ready(letsStart);

The "ready()" method kicks off when all the DOM has (pretty much) loaded, but excanvas.js still wasn't loaded. The solution was to replace "$(document).ready(letsStart);" with

window.onload = letsStart;

Then my canvas app started working fine with an old version of IE.
BTW, my five year old daughter made this with www.MyDoodleDot.com