Answer by Martin Liversage for Will ConfigurationManager.AppSettings["blah"] throw an exception if "blah" doesn't exist?

From the MSDN documentation for NameValueCollection.Item Property (String):

Caution

This property returns null in the following cases: 1) if the specified key is not found; and 2) if the specified key is found and its associated value is null. This property does not distinguish between the two cases.