MPTtree Model: Database Error

[eluser]lightnb[/eluser]
I'm trying to use the MPTtree Tree Traversal library from Martin Wernstahl.

I've got this in my controller:

Code:

$
this->load->model('mpttree', 'MPTtree');
$this->MPTtree->set_opts(array( 'table' => 'Forums_Categories', 'left' => 'LeftID',
                       'right' => 'RightID', 'id' => 'UID',
                       'title' => 'categhoryname'));
$tree = $this->MPTtree->tree2array();
print_r($tree);

and I'm getting the error:

SELECT * FROM Forums_Categories WHERE LeftID BETWEEN AND ORDER BY LeftID ASC

Any ideas?