Getting the z-index of a DIV in JavaScript?

I have a div normaldiv1 with class normaldiv. I am trying to access its z-index through the style property but it always returns 0 although it's set to 2 in the stylesheet.

CSS:

.normaldiv
{
    width:120px; 
    height:50px; 
    background-color:Green;  
    position: absolute;
    left: 25px;
    top:20px;
    display:block;
    z-index:2;
}

`

HTML:

<div id="normaldiv1" 
     class="normaldiv" 
     newtag="new" 
     tagtype="normaldiv1" 
     onmousedown="DivMouseDown(this.id);" 
     ondblclick="EditCollabobaTag(this.id,1);" 
     onclick="GetCollabobaTagMenu(this.id);" 
     onblur="RemoveCollabobaTagMenu(this.id);">

JavaScript:

alert(document.getElementById('normaldiv1').style.zIndex);

How can I find the z-index of an element using JavaScript?