What is wrong with this code? Why the language doesn't load?

[eluser]ReyPM[/eluser]
Hi every:
I'm trying to load a language files but I'm getting this error:

A PHP Error was encountered
Severity: Notice
Message: Undefined property: User::$lang
Filename: controllers/user.php
Line Number: 6

Where line number 6 is this:

Code:

5  public function __construct() {
6        $this->lang->load('common');
7        $this->lang->load('user');
8  }

What's wrong here?
Cheers and thx