Canvas to Canvas Copy Fail

[Ed]

I'm trying to copy from one canvas to another and i get:

uncaught exception: The type of an object is incompatible with the expected type of the parameter associated to the object (NS_ERROR_DOM_TYPE_M

I have 2 canvas dom objects and my code is:

var ctx = document.getElementById('canvas').getContext('2d');
var gtx = document.getElementById('gridbox').getContext('2d');

ctx.drawImage( gtx, 0, 0 );

both canvases are 500x400. The second canvas gtx has a grid of lines i've draw. I can see the grid drawn next to the main canvas, but it won't load it. I'm testing on FF3.

Anyone have any ideas/suggestions?

[Ed]

Fixed. I was passing it the context of the second canvas and not just the dom object itself.

WOrking:

var ctx = document.getElementById('canvas').getContext('2d');
var gb = document.getElementById('gridbox');

ctx.drawImage( gb, 0, 0 );