Report sales by category of products has divide by zero error

If some sales order lines have negative quantities, then the total units sold for a month in a particular category and sales order state can be zero. If the total is zero, then you can get a divide by zero in the Sales by Category of Products report that is part of the report_sale module.

Detailed steps to reproduce:
1. Create a new database with demo data. Select the manufacturing industry profile and leave all the other settings as the defaults.
2. Install the report_sale module.
3. Create a new sales order.
4. Set the customer to Agrolait and the date ordered to one month ago.
5. Add a new sales order line with all the default fields, plus Product Uom PCE, description "Paper plate", and unit price 2.00.
6. Add a second line with all the default fields, plus Quantity UoM -1.00, Product Uom PCE, description "Napkin", and unit price 1.00.
7. Confirm the sale order.
8. From the Sales menu, choose Reporting: All Months: Sales by Category of Products.

Expected behaviour: see the chart.

Actual behaviour: Division by zero error.

I am running OpenERP 5.0.15 on Ubuntu 10.04.