'where子句'中的未知列'IS NULL LIMIT 15'

I am getting an database error i.e Error Number: 1054

Unknown column ' IS NULL LIMIT 15' in 'where clause' SELECT * FROM (categories) WHERE IS NULL LIMIT 15

function categories()
{   

    $this->suri=3;
    $this->surl=site_url('variables/categories/');
    $this->load->library('pagination');
    $cond = array(
                'parent_id' => '0'
                );
    $config['base_url'] = $this->surl;
    $config['total_rows'] = $this->commonmodel->getcountcond('categories',$cond);
    $config['per_page'] = '15';
    $config['first_link'] = 'First';
    $config['last_link'] = 'Last';
    $config['cur_tag_open'] = '<span>';
    $config['cur_tag_close'] = '</span>';
    $config['uri_segment'] = $this->suri; 
    $this->pagination->initialize($config);
    $parent_cond = array(
                'parent_id' => '0'
                 );

I think your query must be:
you must have a reference field in your where clause

SELECT * FROM (categories) WHERE `categories.field` IS NULL LIMIT 15

$this->commonmodel->getcountcond('categories',$cond);

This counter doesn't work.. check it ;)