how to use IS NULL?

Hi,

I have problem with my query, its not update my table

*I want select NULL col with limit then update the DB.

this is my code model:

PHP Code:

function SetUserTakenStock($data, $id, $totalTaken)
  {
    $this->db->trans_start();
    $this->db->select('*');
    $this->db->from('user_id');
    $this->db->where('status IS NULL');
    $this->db->set('status', $data);
    $this->db->where('user_id', $id);
    $this->db->limit($totalTaken);
    $this->db->update('gen_set');

    $this->db->trans_complete();
    if ($this->db->trans_status() === TRUE)
    {
      return true;
    }
    else
    {
      return false;
    }
  } 


This is me. JK not me.