Analytics Aggregates: intervalGranularity 'custom' returns empty results

Using the JS SDK to fetch conversation aggregates with intervalGranularity: 'custom'. The request completes successfully (200 OK), but the intervals array is always empty. I’ve verified the date range covers active data. The JSON payload looks valid, but the API seems to ignore the custom boundaries. Here’s the query object being passed to postAnalyticsConversationsQueuesAggregates.

{"intervalGranularity": "custom", "intervalStart": "2023-10-01T00:00:00.000Z", "intervalEnd": "2023-10-01T23:59:59.999Z"}

Is there a minimum duration requirement or a timezone offset issue I’m missing?