flexi cart - A comprehensive shopping cart library for CodeIgniter

[eluser]AlwynW[/eluser]
While I was looking forward to using your library I can't make it work. I installed all the files correctly (demo files running fine as well) and just want to add an item to the cart and that's where it goes wrong.
I'm using this code in my controller:

Code:

$cart_data = array(
                'id' => $item_id,
                'name' => $name,
                'quantity' => $quantity,
                'price' => $price
            );

       $this->flexi_cart->insert_items($cart_data);

And this is the error:

Message: Undefined index: internal_price
Filename: models/flexi_cart_model.php
Line Number: 771

Which is a variable that according to the documentation is calculated automatically.
This is an exact copy of the code from the demo. What am I doing wrong?