Trying to subscribe to a push topic gives unknown client

Question

Trying to use the subscribe button in the streaming api tool gives an error.

Unknown exception occurred. Removing offending listener or subscription.
[
  {
    "advice": {
      "interval": 0,
      "reconnect": "handshake"
    },
    "channel": "/meta/subscribe",
    "id": "8",
    "error": "403::Unknown client",
    "successful": false
  },
  "Invalid argument: expected subscription, not undefined"
]
8. Subscription Failure: 403::Unknown client
{
  "advice": {
    "interval": 0,
    "reconnect": "handshake"
  },
  "channel": "/meta/subscribe",
  "id": "8",
  "error": "403::Unknown client",
  "successful": false
}