Analytics API interval aggregation returning empty results

Trying to build a custom report using the Conversations Aggregates endpoint. I’ve set the group_by to interval with a 5-minute window, but the response body comes back completely empty. The query runs fine in the UI, so the filters should be valid. Here’s the payload I’m sending via Python:

{"group_by":["interval"],"interval":{"size":"PT5M"},"filters":{"metric":"conv.duration"}}

What am I missing here?