Why does this setting cause 400 in Analytics Conversations Aggregates?

Why does this setting return 400 Bad Request when I use the groupBy parameter?

POST /api/v2/analytics/conversations/aggregates/query
{
 "groupBy": ["medium"],
 "interval": "P1D"
}

“The interval must align with the granularity of the groupBy dimensions.”

Doc says it works, but it fails.