Mocking child actor in Akka

I'm trying to write unit tests for my actor and am stuck on basic mocking. PriceAggregateActor is using akka persistence and I don't want to pass in all the conf for it and would like to mock it completely.

This is the actor that I want to test

object CommandPriceActor {
  def apply() = Props(classOf[CommandPriceActor], PriceAggregateActor())
}

class CommandPriceActor(priceAggregateActorProps: Props) extends Actor with ActorLogging {

  val priceAggregateActor = context.actorOf(priceAggregateActorProps, "priceAggregateActor")

So in my tests I'm trying to do something like:

class CommandPriceActorTest extends TestKit(ActorSystem("test-benefits",
  ConfigFactory.parseString("""akka.loggers = ["akka.testkit.TestEventListener"] """))) with FlatSpecLike with Matchers
  with BeforeAndAfterAll with Eventually{

  class MockedChild extends Actor {
    def receive = {
      case _ => lala
    }
  }

  val probe = TestProbe()
  val commandPriceActor = TestActorRef(new CommandPriceActor(Props[MockedChild]))

I'm always getting:

Caused by: java.lang.IllegalArgumentException: no matching constructor found on class CommandPriceActorTest$MockedChild for arguments []

Why is it complaining about mockedChild? It shouldn't take any constructor arguments.