funktion virker ikke helt

når jeg tilføjer følgende "expand_img" i min funktion fucker den up :( hvad er der galt?

var expand = new Array();
function thread_expand(thread)
{
    if('undefined' == typeof expand[thread]) expand[thread] = true;
   
    var thread_img = document.getElementById('thread_img'+thread);
    var expand_img = document.getElementById('expand_img'+thread);
    var div = document.getElementById('div'+thread);
    var td = document.getElementById('td'+thread);
   
    if(expand[thread])
    {
        thread_img.src = 'gfx/obj.debate_thread_opened.gif';
        expand_img.src = 'gfx/icon.contract.gif';
        div.style.display = 'block';
        td.style.backgroundImage = "url('gfx/obj.debate_thread_bg.gif')";
    }
    else
    {
        thread_img.src = 'gfx/obj.debate_thread_closed.gif';
        expand_img.src = 'gfx/icon.expand.gif';
        div.style.display = 'none';
        td.style.backgroundImage = '';
    }
   
    expand[thread] = !expand[thread];
}