Dojo Tree View Node Collapse SSCE

<html>
    <head>
        <link rel="stylesheet" type="text/css" href="http://ajax.googleapis.com/ajax/libs/dojo/1.6.1/dijit/themes/claro/claro.css"/>
        <script src="http://ajax.googleapis.com/ajax/libs/dojo/1.6.1/dojo/dojo.xd.js"></script>
        <script type="text/javascript">
            dojo.require("dojo.data.ItemFileReadStore");
            dojo.require("dijit.Tree");

            var dojo_data = {identifier: 'id', label: 'text', items: [
                 {id:'-1', text:'root', children:[
                     {_reference:'0'}
                    ,{_reference:'1'}
                    ,{_reference:'2'}]}
                ,{id: '0', text:'Zero', forestRoot: true, children: [
                    {_reference:'3'}]}
                ,{id: '1', text:'One', forestRoot: true, children: [
                    {_reference:'4'}]}
                ,{id: '2', text:'Two', forestRoot: true, children: [
                    {_reference:'5'}]}
                ,{id: '3', text:'Three'}
                ,{id: '4', text:'Four'}
                ,{id: '5', text:'Five'}
            ]};

            dojo.ready(function ( ) {
		var data_store = new dojo.data.ItemFileReadStore({data:dojo_data});
		var tree_model = new dijit.tree.ForestStoreModel({
			rootId: '-1',
			store: data_store,
			query: {forestRoot: true},
			childrenAttrs: ["children"]});
		tree_view = new dijit.Tree(
			{model: tree_model, showRoot: false, autoExpand: true},
			'tree_view_container');
            });

            function collapse ( )
            {
                tree_view.rootNode.getChildren()[0].collapse();
            }

            function expand ( )
            {
                tree_view.rootNode.getChildren()[0].expand();
            }
        </script>
    </head>
    <body class="claro">
        <p style="width: 30em;">Click the Collapse button.  Now click the expand icon next to the collapsed node.
        Note that it will not expand.  Now click the Expand button.  Now click the
        expand icon next to the node multiple times.  Note that it now behaves
        correctly.</p>
        <input type="button" name="Collapse" value="Collapse" onclick="collapse();"/>
        <input type="button" name="Expand" value="Expand" onclick="expand();"/>
        <div id="tree_view_container"></div>
    </body>
</html>