Analytics Aggregates API returning empty results for interval query

Hit a weird wall with the /api/v2/analytics/conversations/aggregates endpoint. I’m trying to pull data for a 15-minute interval, but the response is always empty even though I know there was traffic. Here’s the payload I’m sending:

{
 "groupBy": ["time"],
 "interval": "PT15M",
 "metrics": ["total.handleTime"],
 "filter": {
 "type": "conversation",
 "dimensions": {
 "type": "queue",
 "id": "12345"
 }
 }
}

The API returns a 200 OK, just no data rows. Am I missing a required field or is the interval format wrong?