Problems using HTML Canvas & Greasemonkey not able to drawImage() on 2D context

In my Greasemonkey script, when I obtain a handle on an HTMLImageElement I want to use with an HTML Canvas, I get the following error in Firefox's Error Console (I assume it's because it's enclosed in an XPCNativeWrapper):

Error: Component returned failure code: 0x80040111 (NS_ERROR_NOT_AVAILABLE) 
       [nsIDOMCanvasRenderingContext2D.drawImage]

Putting GM_log() statements throughout my code, I have traced the image object I'm trying to use from it's initial assignment through until I try to use it with an HTML Canvas.

[object XPCNativeWrapper [object HTMLImageElement]]

I've unwrapped the HTMLImageElement by obtaining reference to it with image.wrappedJSObject.

My canvas code:

var canvas = document.createElement("canvas");
canvas.width = image.width;
canvas.height = image.height;

var ctx = canvas.getContext("2d");
ctx.drawImage(image, 0, 0);

Any ideas why Firefox is throwing the above component failure code?

I should have looked more thoroughly on Google.

image.wrappedJSObject;

Works.