Javascript issue on ie

[amitabh joshi]

Varien.Tabs = Class.create();
Varien.Tabs.prototype = {
  initialize: function(selector) {
    var self=this;
    $$(selector+' a').each(this.initTab.bind(this));
  },

  initTab: function(el) {
      el.href = 'javascript:void(0)';
      if ($(el.parentNode).hasClassName('active')) {
        this.showContent(el);
      }
      el.observe('click', this.showContent.bind(this, el));
  },

  showContent: function(a) {
    var li = $(a.parentNode), ul = $(li.parentNode);
    ul.select('li', 'ol').each(function(el){
      var contents = $(el.id+'_contents');
      if (el==li) {
        el.addClassName('active');
        contents.show();
      } else {
        el.removeClassName('active');
        contents.hide();
      }
    });
  }
}
new Varien.Tabs('.tabs');

I am facing problem. I have written above code.



In FireFox and IE8 it works but in IE6-7 it gives me following error.

parent node is null