How to call a config item

[eluser]satie[/eluser]
Why if I use

Code:

$var  = $this->config->item('encryption_key');

CI returns
PHP Fatal error: Call to a member function item() on a non-object ...

The documentation says:
To retrieve an item from your config file, use the following function:

Code:

$this->config->item('item name');

Thanks