Call to undefined method Builder::affectedRows()

in the documentation - affectedRows();

Displays the number of affected rows, when doing “write” type queries (insert, update, etc.).

My code:

PHP Code:

// Method in model
function editClient($id = '', $FIELDS = [])
{
    $db = $this->db->table('WK_clients');

    $db
        ->where('id', (int)$id)
        ->update($FIELDS);

    $FIELDS['id'] = $id;

    return $db->affectedRows() ? $FIELDS : false;
} 


As a result, I get the error:
Call to undefined method CodeIgniter\\Database\\MySQLi\\Builder::affectedRows()

Please tell me what I'm doing wrong.