Codeigniter - > group_start（）和 - > group_end（）不起作用？

I have this piece of code

    $query = $this->db->select('*')
                        ->from('my_table')
                        ->where('monday', '1')
                        ->where('disabled', '0')
                        // ->group_start()
                            ->where('date >=', '2018-09-08')
                            ->or_where('date', NULL)
                        // ->group_end()
                        ->get();

If i uncomment group_start() and group_end() it stops working. The query I want to create should be something like this

select * from my_table where monday = '1' and disabled = '0' and (date >= '2018-09-08' OR date IS NULL)

I am following this documentation (https://www.codeigniter.com/userguide3/database/query_builder.html#query-grouping) and I don't see anything I am doing wrong...