`member_joined_channel`: Failed to parse to a known event

```
Object (fromList [("ts",String "1526332383.000389"),("channel",String "CREDACTED"),("user",String "UREDACTED"),("event_ts",String "1526332383.000389"),("channel_type",String "C"),("team",String "TREDACTED"),("type",String "member_joined_channel")])
Failed to parse to a known event
Please report this failure to the github issue tracker
```