division by zero error in a request

I wants to make a view giving me some statistics.

I am not sure to understand why something like this

SELECT location_id, (sold_parts_amount_dly + sold_labor_amount_dly) /
(sold_parts_amount_dly + sold_labor_amount_dly) from sales

give me a division by zero error?

If that is not the way to go, should I write a function that I would call
instead?

Thanks,
Bernard