We are using the v201509 API, and this morning the report format has changed on us. Yesterday our Quality Score for some of our newer Ads was reported as '0'. Today these are returning quality score as '--'. The Api Migration Doc suggests this is a new feature (String values will have Null encoded as '--') in v201603 and I believe this was unintentionally changed for the v201509 endpoint.
Examples:
Yesterday - qualityScore='0'
Today - qualityScore='--'