Weird Error: Fatal error: Class ‘CI_Controller’ not found in Codeigniter Framework

It’s a simply weird and misleading error message.  I was working on a client project which was actually based on Codeigniter  (CI) framework version 2.1.3.  Somehow, I did a typo in DB setting located at config/database.php file. But, I got a very weird error message: Fatal error: Class ‘CI_Controller’ not found in /var/www/my/……./system/core/CodeIgniter.php on line 233 …