I am trying to change a src from img tag inside javascript code. My html code is the following

Code:
<div>
	<img id = 'op'  src = '2.png'>
</div>
and script code

Code:
	var image = 1;
		var open = 'images/'+image+'.png';
		console.log(open);
		document.getElementById("op").src  = open;
I got the message in the condole Cannot set property 'src' of null. Ant idea??