Call to a member function on a non-object

[eluser]Muncken[/eluser]
It happens in all my files. I will post an example here:

class Groups extends Controller
{
......
.....
.....

function manage()
{
$this->load->model('groups_model');
if ($this->uri->segment(3) == "delete")
{
$this->groups_model->delete();
}
}
}

This gives me:

Fatal error: Call to a member function delete() on a non-object in /Applications/XAMPP/xamppfiles/htdocs/cms/system/application/controllers/groups.php on line 16