TypeError: Attempted to assign to read only property, bootstrap tree-toggle in Safari

Question

Below is some HTML for a bootstrap tree-toggle element. You can see that the second sub list contains the class closed.

<ul id="tree_menu" class="tree">
  <li>
    <input id="74" type="checkbox" data-name="superheros" name="tree_menu_radio" checked="checked">
    <a class="tree-toggle" href="#" data-role="branch" data-toggle="branch">superheros</a>
    <ul class="branch in">
      <li>
        <input id="75" type="checkbox" data-name="first_appeared_in_issue" name="tree_menu_radio">
        <a class="tree-toggle closed" href="#" data-role="branch" data-toggle="branch">first_appeared_in_issue</a>
      </li>
    </ul>
  </li>
</ul>

This works in firefox however in Safari expand works however contract throws the error when you try to contract an element (expand works):

[Error] TypeError: Attempted to assign to readonly property.
    (anonymous function) (bootstrap-tree.js, line 420)
    dispatch (jquery-1.8.3.min.js, line 2)
    u (jquery-1.8.3.min.js, line 2)