Switch Problem

I have some function to Preload images and make MouseOver etc..

But when a put a Switch statement in one of my function (MouseOver), a
receive an error on body load in my preload function.. did someone have any
idea why ?

Here's my switch code :

function setOver(num)
{
var txt;
obj = eval('document.getElementById("image' + num + '")');
obj.src = imagesHover[num-1];

txt = '<img src="' + imagesInfo[num-1] + '">'
document.all("infoDiv").innerHTML=txt;
document.all["infoDiv"].style.display='block'

Switch (num)
{
Case 1: alert(num);Break;
Default: alert("hello");Break;
}
}