POST /api/v2/conversations/conferences/{id}/disconnect returns 400

Trying to kick a participant out of an active conference call via the API. The docs say to POST to /api/v2/conversations/conferences/{id}/disconnect. I’m sending the participant ID in the body, but I keep hitting a 400 Bad Request.

{
 "participantId": "12345678-abcd-1234-abcd-123456789abc"
}

The error message is just “Invalid request” with no details. Is the endpoint deprecated or am I missing a required header?