Forcing a server error in a test for my custom HttpErrorHandler

I've created a custom error handler for a scala play application. For that, I implemented the 2 methods: onClientError and onServerError. I created an integration test and I was able to test onClientError, but I can't find a way to force the server error. This is the test:

class MyJsonHttpErrorHandlerTest extends FeatureSpec
  with ServiceAcceptanceTestSuite
  with PlayService
  with FakeApplicationFactory
  with Matchers
  with WithApplicationComponents {

  override def components: BuiltInComponents = new BuiltInComponentsFromContext(context) with NoHttpFiltersComponents {

    import play.api.mvc.Results
    import play.api.routing.Router
    import play.api.routing.sird._

    lazy val router: Router = Router.from({
      case GET(p"/clienterror") => defaultActionBuilder { Results.Forbidden }
      case GET(p"/servererror") => throw InternalServerErrorException
    })

    val optionalSourceMapper: OptionalSourceMapper = new OptionalSourceMapper(Some(devContext.get.sourceMapper))

    override lazy val httpErrorHandler: HttpErrorHandler = new MyJsonHttpErrorHandler(environment, optionalSourceMapper)
  }

  feature("StudentOptionsJsonHttpErrorHandler test") {

    scenario("Should use our custom error handler for a client error") {

      import play.api.test.Helpers.GET
      import play.api.test.Helpers.route
      val Some(result: Future[Result]) = route(app, FakeRequest(GET, "/clienterror"))

      val json: JsValue = Helpers.contentAsJson(result)
      val requestId = (json \ "error" \ "requestId" ).as[Int]
      val message = (json \ "error" \ "message" ).as[String]
      val uuid = (json \ "error" \ "uuid" ).as[String]

      requestId must be(1)
      message.isEmpty must be(true)
      uuid.matches("[0-9a-fA-F]{8}\\-[0-9a-fA-F]{4}\\-[0-9a-fA-F]{4}\\-[0-9a-fA-F]{4}\\-[0-9a-fA-F]{12}") must be(true)
    }

    scenario("Should use our custom error handler for a server error") {

      import play.api.test.Helpers.GET
      import play.api.test.Helpers.route
      val Some(result: Future[Result]) = route(app, FakeRequest(GET, "/servererror"))

      val json: JsValue = Helpers.contentAsJson(result)
      val requestId = (json \ "error" \ "requestId" ).as[Int]
      val message = (json \ "error" \ "message" ).as[String]
      val uuid = (json \ "error" \ "uuid" ).as[String]

      requestId must be(2)
      message.isEmpty must be(true)
      uuid.matches("[0-9a-fA-F]{8}\\-[0-9a-fA-F]{4}\\-[0-9a-fA-F]{4}\\-[0-9a-fA-F]{4}\\-[0-9a-fA-F]{12}") must be(true)
    }

  }


}

So, when I do a request to /servererror, I can see my custom error handler is going to onClientError. How can I force a fake server error so the onServerError of my handler is invoked?