Analytics API: Grouping by queue and media type returns empty

Trying to build a simple aggregation query for queue wait times, but grouping by both queue.id and mediaType yields zero results. The docs say it’s supported, yet the response is blank. Here’s the payload I’m sending to /api/v2/analytics/queues/summary:

{
 "groupBy": ["queue.id", "mediaType"],
 "interval": "P1D",
 "select": ["offerCount"]
}

Removing mediaType works fine. Adding it back breaks everything. Is this endpoint bugged or am I missing a flag?