All prawns contain tiny amounts of carbs

The law allows producers to round down amounts of carbs to the nearest whole number. So something where the label states it has 0g of carb per 100g could legally have anything between 0 and 1 g per 100g. If you're really concerned to keep accurate count, then you may be safest to count 100g of prawns as 1 g of carbs in the same way that Atkins recommends counting 1 oz of hard cheese as 1 g of carb

If you're less concerned and are eating no more than 100g of prawns a day, enjoy the free ride