I get the following error as soon as I try to checkout with a coupon that makes the product free. The order is placed, but you get an ugly error.
EntityMetadataWrapperException: Invalid data value given. Be sure it matches the required data type and format. in EntityDrupalWrapper->set() (line 737 of /sites/all/modules/entity/includes/entity.wrapper.inc).
If somebody knows how I can fix this, please let me know!