Javascript does not support .length on objects

The functionality on Drupal.quicktabs.tab() does not seem to function correctly for me on a Drupal Commons install with jQuery 1.5.2 installed (fyi, commons is using an unmodified 7.x-3.6 version of the module).

Specifically:

// Constructor for an individual tab
Drupal.quicktabs.tab = function (el) {
  this.element = el;
  this.tabIndex = el.myTabIndex;
  var qtKey = 'qt_' + el.qt_name;
  var i = 0;
  for (var i = 0; i < Drupal.settings.quicktabs[qtKey].tabs.length; i++) {
// ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
// this for loop is never used, because Drupal.settings.quicktabs[qtKey].tabs
// is an object, and .length does not appear to work on objects.
    if (i == this.tabIndex) {
      this.tabObj = Drupal.settings.quicktabs[qtKey].tabs[i];
      this.tabKey = i;
    }
  }
  this.tabpage_id = 'quicktabs-tabpage-' + el.qt_name + '-' + this.tabKey;
  this.container = $('#quicktabs-container-' + el.qt_name);
  this.tabpage = this.container.find('#' + this.tabpage_id);
}

On my install, I could not get tabs to do anything during a click event, and tracing through the stack I can see that the tabpage_id and tabpage were being set to undefined, because the tabObj and tabKey were never set correctly.