Using an external .config file in configSource produces error

I was playing around with how to use the Configuration Manager to read/write custom sections in the App.config file for a WPF application in C#. I read this excellent article on .NET 2.0 Configuration Demystified and it helped me a lot in using the config files. Here is the initial App.config file which I wrote and it works fine.

App.config

But when I changed the App.config file in such a way that my custom section will be read from an external config file mentioned in configSource, Visual Studio gives me an error

The format of a configSource file must be an element containing the name of the section.

Here are the App.config and example.config files

Changed App.config

example.config