Quick JavaScript help?

I'm trying to use JS to check if a string is on a page, and if it is, to inject some HTML into a DIV on the page. Here's my code—what stupid mistake am I making?

Code:

<script type="text/javascript">
  if (document.body.innerHTML.indexOf('My String'))
  	{
  	document.getElementById('target').innerHTML = 'Hello, world!';
  	}
</script>
<div id="target"></div>