When you guys are unit testing an application that relies on values from an app.config file? How do you test that those values are read in correctly and how your program reacts to incorrect values entered into a config file?

It would be ridiculous to have to modify the config file for the NUnit app, but I can't read in the values from the app.config I want to test.

Edit: I think I should clarify perhaps. I'm not worried about the ConfigurationManager failing to read the values, but I am concerned with testing how my program reacts to the values read in.