add_theme_support( 'admin-bar' ) causes fatal error

Question

I am trying to learn more on Theme Development so I've created my own and everything worked fine except when I added functions.php and tried to update it with something simple as:

<?php
 add_theme_support('admin-bar', array('menus'));
?>

I get Server 500 ERROR and I cannot access any part of Wordpress, not even Dashboard. But then as soon as I delete functions.php and refresh page my Wordpress is back again and working smooth.

What is so mysterious about functions.php????

Thank you so much.