Akka-http: how to mock ActorMaterializer in unit test

I have a class A(httpClient: HttpExt)(implicit val system: ActorSystem, materializer: ActorMaterializer) that has a method called extractInfo (res: Future[HttpResponse]): Int that takes a future response and extracts an int value from the response entity.

I want to write a unit test for this method and I have the following in my unit spec class:

class ASpec extends UnitSpec with MockFactory {
  "An A" - {
    implicit val as = mock[ActorSystem]
    implicit val mat = mock[ActorMaterializer]
    val stubHttpClient = stub[HttpExt]
    val a = new A(stubHttpClient)

    "return auth token from response" in {
      val futureResponse: Future[HttpResponse] = <code that returns a Future[HttpResponse]>
      val info: Future[Option[Int]] = A.extractInfo(futureResponse)
      val result = Await.result(info, 10.seconds)
      result shouldBe Some(<a int value>)
    }
  }
}

However, on the line mock[ActorMaterializer], I got the following compilation error:

Error:(19, 28) object creation impossible, since:
it has 3 unimplemented members.
/** As seen from <$anon: akka.stream.ActorMaterializer>, the missing . 
signatures are as follows.
*  For convenience, these are usable as stub implementations.
*/
private[package akka] def actorOf(context: akka.stream.MaterializationContext,props: akka.actor.Props): akka.actor.ActorRef = ???
private[package akka] def logger: akka.event.LoggingAdapter = ???
private[package akka] def supervisor: akka.actor.ActorRef = ???
implicit val mat = mock[ActorMaterializer]

I would love some suggestions on how to solve this problem. Thank you enormously in advance!