This Javascript code draw part of the image on the canvas:

```
var img = document.getElementById('img');
var canvas = document.getElementById('can');
//canvas.width = img.width;
//canvas.height = img.height;
var ctx = canvas.getContext("2d");
ctx.drawImage(img, 0, 0);

```

But when I uncomment the middle lines to set the canvas width and height, the drawImage() does not work.
What should I check in order to find the problem?