IController handler class cannot be loaded

I am trying to create a base controller class but I am receiving this error: "IController handler class cannot be loaded". Any ideas why this is happening?

edited Jun '14

I had almost the exact same error for hours untill I realized the filename of the class file was wrong. When I changed the first letter to a capital letter it suddenly worked. My guess is that you have a problem with the file path.

I checked all of the file paths and I have made no mistakes. Thanks for response nonetheless.