JavaScript - documnet es nulo o no es un objeto

Hola, tengo el siguiente script de java en pagina html y no logro que funcione.
Que puede ser?! saludos! y gracias!

<script type="text/JavaScript">
<!--
function showHPie(){
document.menuPie.src="design/home_pie_over.png"}
function showEHPie(){
document.menuPie.src="design/hotel_pie_over.png"}
function showHaPie(){
document.menuPie.src="design/hab_pie_over.png"}
function showRPie(){
document.menuPie.src="design/ros_pie_over.png"}
function showCPie(){
document.menuPie.src="design/cont_pie_over.png"}
function hideMenuPie(){
document.menuPie.src="design/menu_pie.png"}
</script>

<img class="piepagina" src="design/menu_pie.png" id="menuPie" alt="Menu Pie" border="0" usemap="#pieMap"/>

<map name="pieMap">
<area alt="Home" onMouseOver="showHPie()" onmouseout="hideMenuPie()" shape="rect" coords="1,1,43,13" href="index.html" />
<area alt="El Hotel" onMouseOver="showEHPie()" onmouseout="hideMenuPie()" shape="rect" coords="50,1,109,13" href="hotel.html" />
<area alt="Habitaciones" onMouseOver="showHaPie()" onmouseout="hideMenuPie()" shape="rect" coords="120,1,208,15" href="habitaciones.html" />
<area alt="Rosario desde Altea" onMouseOver="showRPie()" onmouseout="hideMenuPie()" shape="rect" coords="221,1,357,14" href="rosario.html" />
<area alt="Contactanos" onMouseOver="showCPie()" onmouseout="hideMenuPie()" shape="rect" coords="369,0,454,12" href="contacto.html" />
</map>