Noob-Question about subclass prefix

[eluser]Unknown[/eluser]
Hello,

i started just one houre ago with CI. I have little problem at the Section "Creating Libraries" at "General Topics".

Creating the MY_email.php with following code

Code:

class MY_Email extends CI_Email {

    function My_Email()
    {
        parent::CI_Email();
    }
}

does not work. The given Error-Message is

Code:

class MY_Input extends CI_Input { function My_Input() { parent::CI_Input(); } }
Fatal error: Class 'MY_Input' not found in C:\xampp\htdocs\CodeI\system\codeigniter\Common.php on line 123

Why is that happen if i use

Code:

$config['subclass_prefix'] = 'MY_';

?

Thanks
Peter