Can not find div by javascript

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ASP.NET

Javascript

Hi all.

I have a javascript function which adjusts the column height(div) at runtime.

An exception is thrown saying object required.

Following is my function...

function AdjustColumnsHeight() {
    // get a reference to the two DIVS that make up the columns
    var centerCol = window.document.getElementById('centercol');
    var leftCol = window.document.getElementById('leftcol');
    // calculate the max height

    var hCenterCol = centerCol.offsetHeight;
    var hLeftCol = leftCol.offsetHeight;
    var maxHeight = Math.max(hCenterCol, hLeftCol);

    // set the height of all 2 DIVS to the max height
    centerCol.style.height = maxHeight  + 'px';
    leftCol.style.height = maxHeight  + 'px';
      }

And Following is my Master Page..

<div id="container">

        <div id="leftcol">
            <leftMenu:Leftmenue ID="leftMenu1"  runat="server" />
        </div>

        <div id="container2">
            <div id="centercol">
<asp:ContentPlaceHolder id="MainContent"  runat="server">      /asp:ContentPlaceHolder>
            </div>
        </div>
</div>

If I put the centercol div first,then leftcol cannot be found and
If I put the leftcol div first,then centercolcannot be found


Please help me I am in puzzled