How to fix impersonatedUser returned as empty object in Google Drive Activity v2 API?

I'm trying to integrate Google Drive Activity API with my application. There are some events received like this:

...
"actors": [
    {
      "impersonation": {
        "impersonatedUser": {}
      }
    }
  ],
...

According to the docs, this empty object is supposed to be an User object. Not sure if it's a bug or there is anything I can do to fix this?