"subscriptions:insert" returns an error

Today I noticed that the subscriptions:insert method of YouTube Data API V3 returns this error:

{
 "error": {
  "errors": [
   {
    "domain": "youtube.subscription",
    "reason": "publisherNotFound",
    "message": "The resource specified by the requests <code>snippet.resourceId</code> property cannot be found."
   }
  ],
  "code": 404,
  "message": "The resource specified by the requests <code>snippet.resourceId</code> property cannot be found."
 }
}

This is the request body that I sent:

{
 "snippet": {
  "resourceId": {
   "channelId": "UCa90xqK2odw1KV5wHU9WRhg"
  }
 }
}

The above request was valid before. But now, it seems that we need a kind property for snippet.resourceId see below:

{
 "snippet": {
  "resourceId": {
   "channelId": "UCa90xqK2odw1KV5wHU9WRhg",
   "kind": "youtube#channel"
  }
 }
}

It would appear that subscribing to channels without this step is regarded as an invalid request.