很简单?提示我image1Id未定义?


<script language="JavaScript" type="text/javascript">
<!--
function changeState()   
{   
    if(parent.document.getElementById('main_m').cols!="140,8,*")   
    {   
        parent.document.getElementById('main_m').cols="140,8,*";   
        parent.document.getElementById('left').style.display="";   
        image1Id.src = "images/left.gif";   
    }   
    else  
    {   
        parent.document.getElementById('main_m').cols = "0,8,*";   
        parent.document.getElementById('left').style.display="none";   
        image1Id.src = "images/right.gif";   
         
    }   
}  //-->
</script>
  调用:<table height="100%" cellspacing="0" cellpadding="0" border="0">    
            <tbody>  
                <tr  >  
                    <td style=" vertical-align: middle;">                      
                   
                        <img id="image1Id" src="images/left.gif"  onclick="changeState();"  alt="关闭/打开左栏"  border="0" />
                        
                    </td>  
                </tr>  
            </tbody>  
        </table>  
IE6这是,为什么报错呢?而且图也没有变换?