public function whereNull($field, $condition = 'NULL', $operator = 'AND')
    {
        if ( ! in_array($condition, ['NULL', 'NOT NULL']) || ! in_array($operator, ['AND', 'OR'])) {
            throw new \InvalidArgumentException("Logical operator");
        }
        // is the first time call where method ?
        if ($this->_where_str == '') {
            $this->_where_str = ' WHERE ';
        } else {
            $this->_where_str .= ' '.$operator.' ';
        }

        $this->_where_str .= self::_wrapRow($field).' IS '.$condition.' ';

        return $this;
    }