Genesys Cloud WebSocket Notification API subscription failing with 403

Trying to subscribe to conversation events via the Notification API WebSocket using Python.

The connection to wss://api.mypurecloud.com/api/v2/notification/websocket succeeds, but sending the subscribe message returns a 403 Forbidden.

Here is the payload:

{"action":"subscribe","topics":["/queue/54321/inbound"]}

The access token has the conversation:view scope. What am I missing?