C# - Adding Property To TreeNode?

I have a webpage that uses a Treeview. In the treeview are nodes and i use the Text and Value property, but i need one more. I need one boolean property called IsFile.

I make the Nodes and add them to the tree programmatically. I have a class Called NavTreeNodes that inherits the TreeNode class and ads this bool.

public class NavTreeNode : TreeNode
{
private bool _IsFile;
public bool IsFile
{
get { return _IsFile; }
set { _IsFile = value; }
}
public NavTreeNode()
{ }
}

And when i make a new TreeNode i use this class. Everything works until i try to get the data from the treeview in the SelectedNodeChanged on the TreeView function.

protected void treeview_Navigation_SelectedNodeChanged(object sender, EventArgs e)
{
TreeNode node = treeview_Navigation.SelectedNode;
NavTreeNode NNode = node as NavTreeNode;
Response.Write(NNode.IsFile.ToString());
}

I get a "Object reference not set to an instance of an object." error when i try this. I cant even get the Value or the Text value using this method.