SQL problem - SOS!

I have to update a record by adding a number to a field within the same record. Unfortunately, Informix SE informs me that I cannot modify a table that is being used in a sub-query. Can anyone suggest a workaround to this that does not require storing the value in a temporary table or using a variable within another language. Below is an example of the code I am using: - update stockm set back_order_qty = (select back_order_qty + 1 from stockm where product = 'WH5204' and warehouse = '02') where product = 'WH5204' and warehouse = '02' Thanks.