PATCH /api/v2/conversations/voice/{id} 400 error on attributes

Trying to update participant attributes mid-call using the Conversations API, but the PATCH request keeps failing with a 400 error. The JSON payload looks correct based on the docs, so I’m not sure what’s breaking it.

{
 "participants": [
 {
 "id": "12345",
 "attributes": {
 "skill": "billing"
 }
 }
 ]
}

Getting this back:

{ "code": "badRequest", "message": "Invalid attribute format" }

What’s wrong with the structure?