org.mockito.exceptions.misusing.WrongTypeOfReturnValue：在单元测试用例上

Question

我的单元测试用例中出现以下异常

org.mockito.exceptions.misusing.WrongTypeOfReturnValue: 
ConstFuture cannot be returned by toString()
toString() should return String

我测试的代码段在这里

class DeviceStoreServiceESRetrySpec extends WordSpec with Matchers {

  // val elasticStore = mock[ElasticsearchStore]
  val esDao = mock[ESDeviceDao]
  implicit val bulkPutPool: FuturePool =
    FuturePool.interruptible(
      Executors.newFixedThreadPool(10)
    )
  val deviceUpdateStatus = DeviceUpdateStatus("abc", "")
  val res1 = BulkResponse(201, "", List.empty, List.empty, List(deviceUpdateStatus))
  val resp1JsonStr: String = res1.asJson.nospaces
  val res2 = BulkResponse(200, "", List.empty, List.empty, List.empty)
  val resp2JsonStr: String = res2.asJson.nospaces
  val deviceRecords = DeviceArbitraries.bulkArbDevice(2)

  when(esDao.binaryManifestBulkIndex(deviceRecords)(bulkPutPool)) thenReturn (Future.value(
    Try(resp2JsonStr)
  ))

  object MockDeviceServiceModule extends TwitterModule {

    @Provides
    @Singleton
    def provideDeviceDataService(): DeviceStoreService = {
      new DeviceStoreService(
        esDao.binaryManifestBulkIndex,
        100,
        StatsReceiverFactory.nullStatsReceiverFactory,
        StorageConfig("DeviceStoreServiceESRetrySpec", 30, 5),
        Map("host" -> "ES"),
        "Elasticsearch Client"
      )
    }
    val injector = TestInjector(MockDeviceServiceModule)
    val deviceStoreService = injector.instance[DeviceStoreService]

    "An DeviceStoreService instance" should {
      "successfully index device events" in {
        deviceStoreService.store(deviceRecords)
        Mockito.verify(esDao, times(10)).binaryManifestBulkIndex(deviceRecords)
      }
    }
  }

}

请在这里任何帮助