PATCH interaction participant attributes 400 Bad Request on live call

Trying to push CRM data into a live voice call using the REST Proxy in . I’m hitting PATCH /api/v2/interactions/{id}/participants/{participantId} with the JSON below, but it keeps failing with a 400 error saying the attribute map is invalid. I’ve verified the interaction is active and the participant ID is correct.

{
“attributes”: {
“crm_id”: “12345”
}
}

What’s the exact schema requirement for the attributes object during an active session?