CI_DB shows up in __autoload()

[eluser]Phil Sturgeon[/eluser]
I have been faffing around with using __autoload() in CodeIgniter this evening and got a weird result.

Using a clean CodeIgniter installation I added a very simple autoload into the autoload.php config file. Just to play!

Code:

function __autoload($class)
{
    echo $class;
}

This showed me a weird result. CI_DB is being echoed out in a clean 1.7.1 install... What is the reason for that?