Open Messaging API 400 on quick reply JSON

Getting 400 Bad Request when POSTing to /api/v2/webmessaging/organizations/{orgId}/guests/{guestId}/messages. The payload matches the doc example exactly.

{
 "type": "quick-reply",
 "content": {
 "title": "Options",
 "quickReplies": [{"label": "Yes", "value": "yes"}]
 }
}

Response says Invalid payload structure. Anyone see this?