ArgumentOutOfRange when adding root level nodes into treeview

I get ArgumentOutOfRange exception when I try to add nodes into a treeview using the code below:

    protec开发者_运维问答ted void Page_Load(object sender, EventArgs e)
    {
        if (!Page.IsPostBack)
        {
            if (Page.User.Identity.IsAuthenticated)
            {
               NavigationTree.Nodes.Add(AuthUserNode);

            }
            else
            {
                NavigationTree.Nodes.Remove(AuthUserNode);
            }
         }
     }

However I am able to add nodes as a child node of some node

    protected void Page_Load(object sender, EventArgs e)
    {
        if (!Page.IsPostBack)
        {
            if (Page.User.Identity.IsAuthenticated)
            {
               NavigationTree.Nodes[0].ChildNodes.Add(AuthUserNode);

            }
            else
            {
                NavigationTree.Nodes.Remove(AuthUserNode);
            }
         }
     }

What am I missing here?

Finally I have found the error. It is thrown when the node is already present at that level. I wish the exception would be more explanatory though.