Scala Akka HTTP casting parameter as java.time.ZonedDateTime

I'm working on a REST service using Akka HTTP (in Scala). I would like a parameter that is passed in to a http get request to be converted to the ZonedDateTime type. The code works fine if I try to use String or Int but fails with a ZonedDateTime type. The code would look like so:

parameters('testparam.as[ZonedDateTime])

Here is the error that I'm seeing:

Error:(23, 35) type mismatch;
 found   : akka.http.scaladsl.common.NameReceptacle[java.time.ZonedDateTime]
 required: akka.http.scaladsl.server.directives.ParameterDirectives.ParamMagnet
          parameters('testparam.as[ZonedDateTime]){

If I add more than one parameter to the list I get a different error:

Error:(23, 21) too many arguments for method parameters: (pdm: akka.http.scaladsl.server.directives.ParameterDirectives.ParamMagnet)pdm.Out
          parameters('testparam.as[ZonedDateTime], 'testp2){

I found this in the documentation when I was researching the problem http://doc.akka.io/japi/akka-stream-and-http-experimental/2.0/akka/http/scaladsl/server/directives/ParameterDirectives.html and I tried the workaround of adding import akka.http.scaladsl.server.directives.ParameterDirectives.ParamMagnet along with using Scala 2.11 but the problem persisted.

Could someone please explain what I'm doing wrong and why the ZonedDateTime type doesn't work? Thanks in advance!

Here is a code snippet that should reproduce the problem I'm seeing

import java.time.ZonedDateTime

import akka.actor.ActorSystem
import akka.http.scaladsl.Http
import akka.http.scaladsl.model._
import akka.http.scaladsl.server.Directives._
import akka.stream.ActorMaterializer

import scala.io.StdIn


object WebServer {
  def main(args: Array[String]) {

    implicit val system = ActorSystem("my-system")
    implicit val materializer = ActorMaterializer()
    // needed for the future flatMap/onComplete in the end
    implicit val executionContext = system.dispatcher

    val route =
      path("hello") {
        get {
          parameters('testparam.as[ZonedDateTime]){
            (testparam) =>
              complete(testparam.toString)
          }
        }
      }

    val bindingFuture = Http().bindAndHandle(route, "localhost", 8080)

    println(s"Server online at http://localhost:8080/\nPress RETURN to stop...")
    StdIn.readLine() // let it run until user presses return
    bindingFuture
      .flatMap(_.unbind()) // trigger unbinding from the port
      .onComplete(_ => system.terminate()) // and shutdown when done
  }
}