Simple javascript code doesn't work

[FatalBreeze]

Hello.

I'm trying to use this simple code to display ads:

<script type="text/javascript">
init_banners();
function init_banners()
{
 document.getElementById('navabr_720x90').innerHTML = "<script language='javascript' type='text/javascript' src='http://blablabla.com/st?ad_type=ad&ad_size=728x90'><\/script>";
}
</script>

(I put this in footer, and the div is in navbar)

But the ads doesn't show, if i don't use the function, just paste the script so the ads do show so i dont think the problem is with the ads. Even if i replace the ads link with a simple plain text it DOES work.

So how can i fix this?

Thanks in advance.