Upgrading Problems WP 3.51 to 3.6 & Solution Used

Using: PHP v5.4

I have been having problems upgrading the core of wordpress from 3.51 to 3.6

The problem i was getting looked like this:

Fatal error: Call to undefined method Core_Upgrader::init() in /home/*REDACTED*/public_html/wp-admin/includes/class-wp-upgrader.php on line 1020

On line 1020 of that file, it is calling functions that were from the WP_Upgrader that it extended, mainly line 1020 $this->init();

Scroll back up to line 35, and you see that it is calling a variable set from the constructor, except we did not initialize this constructor.

Fix: I added this as the first method ( make sense? )of Core_Upgrader

function __construct(){
parent::__construct();
}

tl:dr;
This fixed the problem; anybody shed some more light on why that needed to be added to fix this problem? I have to update some other sites on the same server, and they are having the same issue. Which leads me to believe it might be a PHP issue that i need to tweak something.