PATCH /conversations/conversations/{conversationId}/participants fails with 400 when setting custom attributes

Trying to update participant attributes mid-call via PATCH /api/v2/conversations/conversations/{conversationId}/participants returns a 400 Bad Request. The JSON payload looks correct:

{
 "participants": [
 {
 "id": "abc-123",
 "attributes": {
 "custom_priority": "high"
 }
 }
 ]
}

The error message just says “Invalid request body” with no details on which field failed. Anyone see this?