NodeValue - Text Appearing Above Rather Than Inside Node

After much work, I've come up with some Javascript/DOM that almost works :)

function setblocktitle(id, text)
{
document.getElementById(id).firstChild.nodeValue = text;
}

With my HTML being:
<div id="hits" class="modules">
<h2>Hits</h2>
<div class="blah">
...

The problem is, when calling setblocktitle with setblocktitle("hits", "Totals"); I end up with the attached image rather than "Hits" being replaced by "Totals".