I have defined a custom ToEntityMarshaller for type User. When requesting 
/users it returns an empty JSON array. Only when I remove the implicit def 
userMarshaller it return the correct representation of the stream.

Anybody has a pointer on what is going wrong?

import akka.NotUsed
import akka.actor.ActorSystem
import akka.http.scaladsl.Http
import akka.http.scaladsl.common.{EntityStreamingSupport, 
import akka.http.scaladsl.model.{HttpEntity, StatusCodes, _}
import akka.http.scaladsl.server.Directives._
import akka.stream.ActorMaterializer
import akka.http.scaladsl.marshallers.sprayjson.SprayJsonSupport._
import akka.http.scaladsl.marshalling.{Marshaller, ToEntityMarshaller, 
import akka.http.scaladsl.model.TransferEncodings.gzip
import akka.http.scaladsl.model.headers.{HttpEncoding, HttpEncodings}
import akka.http.scaladsl.model.ws.{Message, TextMessage}
import akka.stream.scaladsl.{Flow, Source}
import akka.util.ByteString
import spray.json.DefaultJsonProtocol
import spray.json.DefaultJsonProtocol._

import scala.concurrent.Future
import scala.io.StdIn
import scala.util.Random

final case class User(name: String, id: String)

trait UserProtocol extends DefaultJsonProtocol {

  import spray.json._

  implicit val userFormat = jsonFormat2(User)

  val `vnd.example.api.v1+json` =

  implicit def userMarshaller: ToEntityMarshaller[User] = Marshaller.oneOf(
    Marshaller.withFixedContentType(`vnd.example.api.v1+json`) { organisation =>
      HttpEntity(`vnd.example.api.v1+json`, organisation.toJson.compactPrint)

object ApiServer extends App with UserProtocol {
  implicit val system = ActorSystem("api")
  implicit val materializer = ActorMaterializer()
  implicit val executionContext = system.dispatcher

  implicit val jsonStreamingSupport: JsonEntityStreamingSupport = 
    .withParallelMarshalling(parallelism = 10, unordered = false)

  // (fake) async database query api
  def dummyUser(id: String) = User(s"User $id", id.toString)

  def fetchUsers(): Source[User, NotUsed] = Source.fromIterator(() => 
Iterator.fill(10000) {
    val id = Random.nextInt()

  val route =
    pathPrefix("users") {
        get {

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

  println(s"Server online at http://localhost:8080/\nPress RETURN to stop...")
  bindingFuture.flatMap(_.unbind()).onComplete(_ => system.terminate())

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