Analytics Aggregates API returning empty data for custom interval report

Trying to build a custom interval report in C# using the .NET SDK (v13) but I’m getting zero data back. The docs say POST /api/v2/analytics/conversations/aggregates should work for this, but the response body is just {"count":0, "results":[]}.

Here is the payload I’m sending via HttpClient:

{
 "groupBy": ["wrap-up-code"],
 "dateFrom": "2023-10-01T00:00:00.000Z",
 "dateTo": "2023-10-01T23:59:59.999Z",
 "select": ["talkDuration"],
 "where": "conversation.type eq 'voice'"
}

I verified the date range has traffic. I can see calls in the admin UI for that exact day. If I change groupBy to [] (empty array) and add interval: 'P1D', it returns data. But the moment I add a groupBy field, it dies.

The docs state: “When using groupBy, you must also specify an interval or use a custom interval report.” I’m not sure what “custom interval report” means in the context of the JSON payload. Is there a specific property I’m missing? I tried adding "interval": "PT1H" but that just broke it further with a 400 error saying interval is not allowed with groupBy.

Am I missing a required field in the request body?