JavaScript Error - Unable to set property 'zIndex' of undefined or null reference

var isNav, isIE
var coll = ""
var styleObj = ""

var zIndex = 2  // Must be larger than our backgrounds

if (parseInt(navigator.appVersion) >= 4) {
  if (navigator.appName == "Netscape") {
    isNav = true
  } else {
    isIE = true
    coll = "all."
    styleObj = ".style"
  }
}

function show(obj) {
  obj.visibility = "visible"
}

function hide(obj) {
  obj.visibility = "hidden"
}

function changeVisibility(Obj,num) {
  var imgobj = eval('document.' + coll + 'image' + num + styleObj)
  
  // If it's one of our backgrounds we don't want to change its
  // zIndex
  //if (num > 22) {
  // if (Obj.checked)
  //    show(imgobj)
  //  else
  //    hide(imgobj)
  // return
  //}
  if (Obj.checked) {
    zIndex++
    setZIndex(imgobj, zIndex)
    show(imgobj)
  } else {
    hide(imgobj)
    zIndex--
  }
}

function setZIndex(obj, zOrder) {
  obj.zIndex = zOrder
}

function openWin(url, name) {
  popupWin = window.open(url, name, 'toolbar=0,location=0,directories=0,status=0,menubar=0,scrollbars=1,resizable=1,width=350,height=120');
}

function openBWin(url) {
 popupWin = window.open(url);
}