Error: document.getElementById(img) has no properties

[pcof]

Hi Folks!
This is my first post in this forum so excuse me any incovenience.

I am getting this error with firefox with the script below:
Error: document.getElementById(img) has no properties

HTML: www.rockhour.com.br/preview (http://www.rockhour.com.br/preview)

<---------------
function MostrarMenu(div)
{
EsconderTudo();
menu = "Menu" + div;
img = "Img" + div;
img_over = "images/btn_" + div + "_over.gif"
document.getElementById(img).src = img_over;
document.getElementById(menu).style.display='';
}
function EsconderMenu(div)
{
menu = "Menu" + div;
img = "Img" + div;
img_out = "images/btn_" + div + "_out.gif"
document.getElementById(img).src = img_out;
document.getElementById(menu).style.display='none';
}
function EsconderTudo()
{
EsconderMenu("festival");
EsconderMenu("galeria");
}
------------>

If anyone may help me. Thanks!

[Twey]

I am getting this error with firefoxI'm not.