Treeview control error when using autopostback

[Stuck]

I have created a new c# class which inherits the Microsoft treeview control. I have set autopostback = true in the constructor of my new class. I have created a new web page and created an instance of my new class and populated it from an XML file using code. I then add my control to the page so that it is displayed. All this works fine, but when a user clicks on a node that has child nodes I get the following error: Object expected. Then when I press break I get a there is no source code for the following location. Can any one suggest why I might be getting this error

All I want to do is beable to access the OnSelectedIndexChange event in my new inherited TreeView class