javascript / canvas5 error from simple code

Error: uncaught exception: [Exception... "Component returned failure code: 0x80040111 (NS_ERROR_NOT_AVAILABLE) [nsIDOMCanvasRenderingContext2D.drawImage]" nsresult: "0x80040111 (NS_ERROR_NOT_AVAILABLE)" location: "JS frame :: http://127.0.11.1/test/canvas5_b.php :: setup :: line 27" data: no]

function setup() {

    var e = document.getElementById("mycanvas");
    var ctx = e.getContext('2d');


    var meter = new Image();
    meter.src = "meter.jpg";
    var meter_bar = new Image();
    meter_bar.src = "meter_bar.jpg";
    //alert(meter);

    ctx.beginPath();/////////LINE 27////////////
    ctx.drawImage(meter, 50, 100);
    //ctx.globalCompositeOperation = "lighter";
    ctx.drawImage(meter_bar, 68, 123);
    ctx.closePath();


}

window.onload = setup;

Both the images are in the right folder. The thing that gets me is that it works if you put a alert(meter); before line 27. Its as if it is not loaded, but I have it running on window.onload, so I dont see how that is.

edit : It is an issue of when the image is loaded (ty rob). It appears best to globally declare and set the image src, and then call window.onload = setup, like this : (correct me if this is bad)

var img1, img2;
img1 = new Image();
img2 = new Image();
//declare and set the images src
img1.src = "meter.jpg";
img2.src = "meter_bar.jpg";

var canvasHeight, canvasWidth;
canvasHeight = 300;
canvasWidth= 600;

var ctx;


function setup() {
    var e = document.getElementById("mycanvas");
    ctx = e.getContext('2d');
    draw();
}

function draw() {
    ctx.clearRect(0,0,canvasWidth, canvasHeight);
    ctx.beginPath();
    ctx.drawImage(img1, 50, 100);
    ctx.drawImage(img2, 68, 123);
    ctx.closePath();
}

window.onload = setup;

Make sure the images are loaded first. For instance:

 var img = new Image();
 img.onload = function(){
    ctx.drawImage(img,0,0);
    }

 // put this after onload, otherwise onload may 
 // not be called if image is in cache
 img.src = 'whatev.png';