Spock mocks for Akka's ActorRef

I've tried to make an Spock test for a class, where i need to check that it sends a message to actor (say statActor). I know that Akka have special library for integration test, but seems that it's too much for very simple test. So, i've tried:

setup:
def myActor = Mock(ActorRef)
myService.statActor = myActor
when:
myService.startStats()
then:
1 * myActor.tell(_)

Target method looks like:

void startStats() {
    Date x = null
    // prepare some data, fill x with required value
    this.statActor.tell(x)
}

I thought that Spock will create mock with a method tell. But after running this test i'm getting:

java.lang.ClassCastException: akka.actor.ActorRef$$EnhancerByCGLIB$$80b97938 cannot be cast to akka.actor.ScalaActorRef
    at akka.actor.ActorRef.tell(ActorRef.scala:95)
    at com.example.MyService.startStats(MyService.groovy:32)

Why it's calling real ActorRef implementation? Some kind of incompatibility with Scala? Is there any way to make mock for such class?