accesing model from browser?

Out of curiosity I tried to open my model through the browser:

http://www.domain.com/application/models/My_model.php

and instead of getting some "not found" or "no access" error I got:

Fatal error: Class 'CI_Model' not found in /home/xxx/public_html/application/models/My_model.php on line 3

is this normal, or did I something wrong during installation?