uploading on subfolders

[eluser]radd[/eluser]
Hi guys,
I'm new here and here's my dull question:

Running CI on my localhost using XAMPP and I'm trying to upload images on a 'uploads' folder but in subfolders (ie. each user has it's own subfolder of uploaded images and thumbs).

SAMPLE:

Code:

$id = $this->session->userdata('id');
$config['upload_path'] = 'uploads/'.$id.'/';

But when I run this, I get a: The upload path does not appear to be valid.
How should I do it?