Query not working, when I try to increment existing record

Question

I am using codeigniter.

Trying to use query (I got this by using get_compiled_update('invoice'))

UPDATE `invoice` SET `paid` = 'paid + 100' WHERE `invoice_id` = '46'

but it is not working, saying

0 rows affected

In my model

 $this->db
            ->where('invoice_id',$invoice_id)
            ->set('paid',"paid + ".$payment_price)
            ->update('invoice');

What should I do to make it work.