SimpleTester 1.0 - CI-library for unit testing

[eluser]geshan[/eluser]
When I try to test a controller function that calls a model, and in case the model calls functions of the DB class like query, insert etc there is an error like below:
<code>
Fatal error: Call to a member function insert() on a non-object in CodeIgniter170\system\application\models\user_model.php on line 39
</code>

How do I test these things eliminating the errors?