Akka Streams fails with NPE using twitter4j-stream

Question

I'm experimenting with Akka streams and twitter4j-stream. Having a stream of tweets it works fine until the moment when it fails with NPE, which doesn't make sense to me.

Initially i'm creating a client:

val client: TwitterStream = {
  using(TwitterClient.clientFactory.getInstance()) { inst =>
    inst.setOAuthConsumer(auth.consumerKey, auth.consumerSecret)
    inst.setOAuthAccessToken(new AccessToken(auth.accessTokenKey, auth.accessTokenSecret))
  }
}

In the end i want to get something with the following signature: val stream: Source[TwitterEvent, ShutdownHandler] so i'm creating a ReactiveStreams Publisher which would feed the Source using the that comes out of the client:

val pub = new Publisher[TwitterEvent] {
  override def subscribe(s: Subscriber[_ >: TwitterEvent]): Unit = {
    client.addListener(new TwitterListener(s))
    client.filter(filter)
  }
}

val stream: Source[TwitterEvent, ShutdownHandler] = {
  Source.fromPublisher(pub).mapMaterializedValue { nu =>
    new TwitterClient.ShutdownHandler(() => client.shutdown())
  }
}

So the .filter() bootstrap this thing and i'm receiving some amout of tweets and then it fails with this:

[ERROR] [04/08/2016 15:18:19.301] [System-akka.actor.default-dispatcher-4] [akka://System/user/StreamSupervisor-0/flow-0-0-foreachSink-foreachSink-map] It is illegal to throw exceptions from request(), rule 3.16
akka.stream.impl.ReactiveStreamsCompliance$SignalThrewException: It is illegal to throw exceptions from request(), rule 3.16
    at akka.stream.impl.ReactiveStreamsCompliance$.tryRequest(ReactiveStreamsCompliance.scala:111)
...
Caused by: java.lang.NullPointerException
    at akka.stream.impl.ReactiveStreamsCompliance$.tryRequest(ReactiveStreamsCompliance.scala:110)

In case it's important my TwitterListener:

final class TwitterListener(s: Subscriber[_ >: TwitterEvent]) extends StatusAdapter {
    override def onStatus(status: Status): Unit = {
      if (status != null) {
        val message = new TwitterEvent(status)
        s.onNext(message)
      }
    }

    override def onException(ex: Exception): Unit = {
      s.onError(ex)
    }
  }