Disconnecting specific participant from conference call via API fails with 400

Trying to drop a specific participant from an active conference using the .NET SDK. The docs say to PATCH the participant’s communication to disconnect it, but it keeps returning a 400 Bad Request.

Here’s the call:

var result = await _conversationsApi.PatchConversationParticipantCommunicationAsync(conversationId, participantId, communicationId, new DisconnectParticipantRequest());

The error payload just says "error": "Invalid request". I’ve verified the IDs are correct by listing participants first. What am I missing?