Error saving packs

[inovlancer]

Hello, community.

 

Whenever I save a pack, this error shows up:

Warning: array_key_exists(): The first argument should be either a string or an integer in /home/******/public_html/classes/Pack.php on line 49 (this error shows up 3 times, it might be because all our client's packs have 3 products)

Warning: Cannot modify header information - headers already sent by (output started at /home/******/public_html/classes/Pack.php:49) in /home/******/public_html/classes/Tools.php on line 252

 

I've searched online for this but I can't seem to find anyone with the same problem or similar.

 

I don't really have much more details to this. Line 49 has this:

if (!array_key_exists($id_product, self::$cacheIsPack)) {
    $result = Db::getInstance()->getValue('SELECT COUNT(*) FROM `'._DB_PREFIX_.'pack` WHERE id_product_pack = '.(int)$id_product);
    self::$cacheIsPack[$id_product] = ($result > 0);
}

 

Some help would be extremely appreciated.

Thanks in advance!

Edited July 11, 2018 by inovlancer (see edit history)