query builder HAVING clause

Hello!
I'm trying to write SQL with query builder(QB). I want to use HAVING clause with MIN() function in it. In Mysql CLI everything works fine, but QB produce string with "IS NULL" in the end.

$this->db->HAVING('MIN(sp.s_price)', null, false);
$this->db->HAVING('MIN(sp.s_price)', '', false);
It's didn't help.

Code:

SELECT `s`.`s_code`, `sp`.`s_price`
FROM `services` `s`
LEFT JOIN `service_prices` `sp` ON `s`.`s_id` = `sp`.`s_id`
WHERE `s`.`s_name` LIKE '%q%' ESCAPE '!'
AND `s`.`s_activity` = '1'
GROUP BY `s`.`s_id`
HAVING MIN(sp.s_price) IS NULL

PHP Code:

$srvcCode = 'q';
$this->db->select('s.s_code, sp.s_price');
$this->db->from('services s');
$this->db->join('service_prices sp', 's.s_id = sp.s_id', 'left');
$this->db->like('s.s_name', $srvcCode);
$this->db->where('s.s_activity', '1');
$this->db->group_by('s.s_id');
$this->db->having('MIN(sp.s_price)');

echo '<pre>';
print_r($this->db->get_compiled_select());
echo '</pre>';
die; 


 
I found workaround $this->db->having("MIN(sp.s_price)", "MIN(sp.s_price)", false);
But It's not a good practice.

Is it a bug or I didn't understand something?

English isn’t my first language, so please excuse any mistakes