CXone Studio: Updating Participant Attributes via REST Proxy during active call

Trying to write a custom attribute to a participant mid-call using GetRESTProxy in . The PUT /api/v2/conversations/voice/participants/{id} call returns 400 Bad Request with INVALID_ARGUMENT. The payload looks right:

{
 "attributes": {
 "custom": {
 "priority": "high"
 }
 }
}

The participant ID is valid. Is there a specific format for the nested attributes object that the REST Proxy requires?