diff --git a/akka-stream-tests/src/test/scala/akka/stream/scaladsl/FlowExpandSpec.scala b/akka-stream-tests/src/test/scala/akka/stream/scaladsl/FlowExpandSpec.scala
index 6c956b7070..de20d38669 100644
--- a/akka-stream-tests/src/test/scala/akka/stream/scaladsl/FlowExpandSpec.scala
+++ b/akka-stream-tests/src/test/scala/akka/stream/scaladsl/FlowExpandSpec.scala
@@ -72,7 +72,7 @@ class FlowExpandSpec extends AkkaSpec {
         .expand[Int, Int](seed = i ⇒ i, extrapolate = i ⇒ (i, i))
         .fold(Set.empty[Int])(_ + _)
 
-      Await.result(future, 10.seconds) should be(Set.empty[Int] ++ (1 to 100))
+      Await.result(future, 10.seconds) should contain theSameElementsAs ((1 to 100).toSet)
     }
 
     "backpressure publisher when subscriber is slower" in {
diff --git a/akka-stream-tests/src/test/scala/akka/stream/scaladsl/FlowIteratorSpec.scala b/akka-stream-tests/src/test/scala/akka/stream/scaladsl/FlowIteratorSpec.scala
index c821db9f3d..56888c0c7b 100644
--- a/akka-stream-tests/src/test/scala/akka/stream/scaladsl/FlowIteratorSpec.scala
+++ b/akka-stream-tests/src/test/scala/akka/stream/scaladsl/FlowIteratorSpec.scala
@@ -37,9 +37,10 @@ class FlowIteratorSpec extends AkkaSpec {
     }
 
     "complete empty" in {
-      val p = Source(List.empty[Int].iterator).runWith(Sink.publisher)
+      val p = Source[Int](Iterator.empty).runWith(Sink.publisher)
       val c = StreamTestKit.SubscriberProbe[Int]()
       p.subscribe(c)
+      c.expectSubscription()
       c.expectComplete()
       c.expectNoMsg(100.millis)
 
diff --git a/akka-stream-tests/src/test/scala/akka/stream/scaladsl/GraphBalanceSpec.scala b/akka-stream-tests/src/test/scala/akka/stream/scaladsl/GraphBalanceSpec.scala
index ba856d1a96..049063b36e 100644
--- a/akka-stream-tests/src/test/scala/akka/stream/scaladsl/GraphBalanceSpec.scala
+++ b/akka-stream-tests/src/test/scala/akka/stream/scaladsl/GraphBalanceSpec.scala
@@ -136,17 +136,14 @@ class GraphBalanceSpec extends AkkaSpec {
 
     "fairly balance between three outputs" in {
       val numElementsForSink = 10000
-      val f1, f2, f3 = Sink.fold[Int, Int](0)(_ + _)
+      val outputs = Seq.fill(3)(Sink.fold[Int, Int](0)(_ + _))
       val g = FlowGraph { implicit b ⇒
         val balance = Balance[Int]("balance", waitForAllDownstreams = true)
-        Source(Stream.fill(10000 * 3)(1)) ~> balance ~> f1
-        balance ~> f2
-        balance ~> f3
+        Source(Stream.fill(numElementsForSink * outputs.size)(1)) ~> balance
+        for { o ← outputs } balance ~> o
       }.run()
 
-      Seq(f1, f2, f3) map { sink ⇒
-        Await.result(g.get(sink), 3.seconds) should be(numElementsForSink +- 1000)
-      }
+      for { o ← outputs } Await.result(g.get(o), 3.seconds) should be(numElementsForSink +- 1000)
     }
 
     "produce to second even though first cancels" in {
diff --git a/akka-stream/src/main/scala/akka/persistence/stream/PersistentSource.scala b/akka-stream/src/main/scala/akka/persistence/stream/PersistentSource.scala
index 4ce7cadfdf..d78167863f 100644
--- a/akka-stream/src/main/scala/akka/persistence/stream/PersistentSource.scala
+++ b/akka-stream/src/main/scala/akka/persistence/stream/PersistentSource.scala
@@ -12,7 +12,6 @@ import akka.stream.impl.Cancel
 import akka.stream.impl.ExposedPublisher
 import akka.stream.impl.RequestMore
 import akka.stream.impl.SoftShutdown
-import akka.stream.impl.Stop
 import akka.stream.impl.SubscribePending
 import akka.stream.impl.SubscriberManagement
 import akka.stream.impl.ActorBasedFlowMaterializer
@@ -112,11 +111,7 @@ private class PersistentSourceImpl(persistenceId: String, sourceSettings: Persis
     case Cancel(sub) ⇒
       unregisterSubscription(sub.asInstanceOf[S])
     case Response(ps) ⇒
-      try {
-        ps.foreach(pushToDownstream)
-      } catch {
-        case Stop ⇒
-          completeDownstream(); shutdownReason = None
+      try ps.foreach(pushToDownstream) catch {
         case NonFatal(e) ⇒ abortDownstream(e); shutdownReason = Some(e)
       }
   }
diff --git a/akka-stream/src/main/scala/akka/stream/impl/ActorPublisher.scala b/akka-stream/src/main/scala/akka/stream/impl/ActorPublisher.scala
index 03294555c3..97ff8ccfad 100644
--- a/akka-stream/src/main/scala/akka/stream/impl/ActorPublisher.scala
+++ b/akka-stream/src/main/scala/akka/stream/impl/ActorPublisher.scala
@@ -17,9 +17,7 @@ import org.reactivestreams.{ Publisher, Subscriber }
  * INTERNAL API
  */
 private[akka] object SimpleCallbackPublisher {
-  def props[T](settings: MaterializerSettings, f: () ⇒ T): Props =
-    Props(new SimpleCallbackPublisherImpl(f, settings)).withDispatcher(settings.dispatcher)
-
+  def props[T](f: () ⇒ T, settings: MaterializerSettings): Props = IteratorPublisher.props(Iterator.continually(f()), settings)
 }
 
 /**
@@ -126,22 +124,23 @@ private[akka] trait SoftShutdown { this: Actor ⇒
 /**
  * INTERNAL API
  */
-private[akka] object SimpleCallbackPublisherImpl {
-  case object Generate
+private[akka] object IteratorPublisherImpl {
+  case object Flush
 }
 
 /**
  * INTERNAL API
  */
-private[akka] class SimpleCallbackPublisherImpl[T](f: () ⇒ T, settings: MaterializerSettings)
+private[akka] class IteratorPublisherImpl[T](iterator: Iterator[T], settings: MaterializerSettings)
   extends Actor
   with ActorLogging
   with SubscriberManagement[T]
   with SoftShutdown {
 
-  import akka.stream.impl.SimpleCallbackPublisherImpl._
+  import IteratorPublisherImpl.Flush
 
   type S = ActorSubscription[T]
+  private var demand = 0L
   var pub: ActorPublisher[T] = _
   var shutdownReason: Option[Throwable] = ActorPublisher.NormalShutdownReason
 
@@ -155,36 +154,46 @@ private[akka] class SimpleCallbackPublisherImpl[T](f: () ⇒ T, settings: Materi
     case SubscribePending ⇒
       pub.takePendingSubscribers() foreach registerSubscriber
       context.become(active)
+      flush()
   }
 
   final def active: Receive = {
     case SubscribePending ⇒
       pub.takePendingSubscribers() foreach registerSubscriber
+      flush()
     case RequestMore(sub, elements) ⇒
       moreRequested(sub.asInstanceOf[S], elements)
-      generate()
+      flush()
     case Cancel(sub) ⇒
       unregisterSubscription(sub.asInstanceOf[S])
-      generate()
-    case Generate ⇒
-      generate()
+      flush()
+    case Flush ⇒
+      flush()
   }
 
   override def postStop(): Unit =
     if (pub ne null) pub.shutdown(shutdownReason)
 
-  private var demand = 0L
-  private def generate(): Unit = {
-    if (demand > 0) {
-      try {
-        demand -= 1
-        pushToDownstream(f())
-        if (demand > 0) self ! Generate
-      } catch {
-        case Stop        ⇒ { completeDownstream(); shutdownReason = None }
-        case NonFatal(e) ⇒ { abortDownstream(e); shutdownReason = Some(e) }
-      }
+  private[this] def flush(): Unit = try {
+    val endOfStream =
+      if (iterator.hasNext) {
+        if (demand > 0) {
+          pushToDownstream(iterator.next())
+          demand -= 1
+          iterator.hasNext == false
+        } else false
+      } else true
+
+    if (endOfStream) {
+      completeDownstream()
+      shutdownReason = None
+    } else if (demand > 0) {
+      self ! Flush
     }
+  } catch {
+    case NonFatal(e) ⇒
+      abortDownstream(e)
+      shutdownReason = Some(e)
   }
 
   override def initialBufferSize = settings.initialFanOutBufferSize
@@ -203,5 +212,4 @@ private[akka] class SimpleCallbackPublisherImpl[T](f: () ⇒ T, settings: Materi
     pub.shutdown(shutdownReason)
     softShutdown()
   }
-
 }
diff --git a/akka-stream/src/main/scala/akka/stream/impl/IteratorPublisher.scala b/akka-stream/src/main/scala/akka/stream/impl/IteratorPublisher.scala
index 4ef9b572d6..d3ec1f32d9 100644
--- a/akka-stream/src/main/scala/akka/stream/impl/IteratorPublisher.scala
+++ b/akka-stream/src/main/scala/akka/stream/impl/IteratorPublisher.scala
@@ -10,12 +10,7 @@ import akka.stream.MaterializerSettings
  * INTERNAL API
  */
 private[akka] object IteratorPublisher {
-  def props(iterator: Iterator[Any], settings: MaterializerSettings): Props = {
-    def f(): Any = {
-      if (!iterator.hasNext) throw Stop
-      iterator.next()
-    }
-    SimpleCallbackPublisher.props(settings, f)
-  }
+  def props[T](iterator: Iterator[T], settings: MaterializerSettings): Props =
+    Props(new IteratorPublisherImpl(iterator, settings)).withDispatcher(settings.dispatcher)
 
 }
\ No newline at end of file
diff --git a/akka-stream/src/main/scala/akka/stream/impl/Stop.scala b/akka-stream/src/main/scala/akka/stream/impl/Stop.scala
deleted file mode 100644
index 55831f62fb..0000000000
--- a/akka-stream/src/main/scala/akka/stream/impl/Stop.scala
+++ /dev/null
@@ -1,15 +0,0 @@
-/**
- * Copyright (C) 2014 Typesafe Inc. <http://www.typesafe.com>
- */
-package akka.stream.impl
-
-import scala.util.control.ControlThrowable
-
-/**
- * INTERNAL API
- *
- * This exception must be thrown from a callback-based stream publisher to
- * signal the end of stream (if the produced stream is not infinite). This is used for example in
- * [[akka.stream.scaladsl.Flow#apply]] (the variant which takes a closure).
- */
-private[akka] case object Stop extends ControlThrowable
diff --git a/akka-stream/src/main/scala/akka/stream/scaladsl/ActorFlowSource.scala b/akka-stream/src/main/scala/akka/stream/scaladsl/ActorFlowSource.scala
index b7711d7644..02cc7658e6 100644
--- a/akka-stream/src/main/scala/akka/stream/scaladsl/ActorFlowSource.scala
+++ b/akka-stream/src/main/scala/akka/stream/scaladsl/ActorFlowSource.scala
@@ -3,20 +3,18 @@
  */
 package akka.stream.scaladsl
 
-import akka.actor.ActorRef
-import akka.actor.Props
+import akka.actor.{ Props, ActorRef }
 import akka.stream.impl._
-import akka.stream.impl.ActorBasedFlowMaterializer
 import akka.stream.impl.Ast.AstNode
 import org.reactivestreams.Publisher
 import org.reactivestreams.Subscriber
 
 import scala.annotation.unchecked.uncheckedVariance
+import scala.annotation.tailrec
 import scala.collection.immutable
 import scala.concurrent.Future
 import scala.concurrent.duration.FiniteDuration
-import scala.util.Failure
-import scala.util.Success
+import scala.util.{ Success, Failure }
 
 sealed trait ActorFlowSource[+Out] extends Source[Out] {
 
@@ -114,8 +112,7 @@ final case class IteratorSource[Out](iterator: Iterator[Out]) extends SimpleActo
     create(materializer, flowName)._1.subscribe(flowSubscriber)
   override def isActive: Boolean = true
   override def create(materializer: ActorBasedFlowMaterializer, flowName: String) =
-    if (iterator.isEmpty) (EmptyPublisher[Out], ())
-    else (ActorPublisher[Out](materializer.actorOf(IteratorPublisher.props(iterator, materializer.settings),
+    (ActorPublisher[Out](materializer.actorOf(IteratorPublisher.props(iterator, materializer.settings),
       name = s"$flowName-0-iterator")), ())
 }
 
@@ -135,21 +132,31 @@ final case class IterableSource[Out](iterable: immutable.Iterable[Out]) extends
       name = s"$flowName-0-iterable")), ())
 }
 
-/**
- * Define the sequence of elements to be produced by the given closure.
- * The stream ends normally when evaluation of the closure returns a `None`.
- * The stream ends exceptionally when an exception is thrown from the closure.
- */
-final case class ThunkSource[Out](f: () ⇒ Option[Out]) extends SimpleActorFlowSource[Out] {
-  override def attach(flowSubscriber: Subscriber[Out], materializer: ActorBasedFlowMaterializer, flowName: String) =
-    create(materializer, flowName)._1.subscribe(flowSubscriber)
-  override def isActive: Boolean = true
-  override def create(materializer: ActorBasedFlowMaterializer, flowName: String) =
-    (ActorPublisher[Out](materializer.actorOf(SimpleCallbackPublisher.props(materializer.settings,
-      () ⇒ f() match {
-        case Some(out) ⇒ out
-        case _         ⇒ throw Stop
-      }), name = s"$flowName-0-thunk")), ())
+final class ThunkIterator[Out](thunk: () ⇒ Option[Out]) extends Iterator[Out] {
+  require(thunk ne null, "thunk is not allowed to be null")
+  private[this] var value: Option[Out] = null
+
+  private[this] def advance(): Unit =
+    value = thunk() match {
+      case null   ⇒ throw new NullPointerException("Thunk is not allowed to return null")
+      case option ⇒ option
+    }
+
+  @tailrec override final def hasNext: Boolean = value match {
+    case null ⇒
+      advance(); hasNext
+    case option ⇒ option.isDefined
+  }
+
+  @tailrec override final def next(): Out = value match {
+    case null ⇒
+      advance(); next()
+    case Some(next) ⇒
+      advance(); next
+    case None ⇒ Iterator.empty.next()
+  }
+
+  override def toString: String = "ThunkIterator"
 }
 
 /**
diff --git a/akka-stream/src/main/scala/akka/stream/scaladsl/Source.scala b/akka-stream/src/main/scala/akka/stream/scaladsl/Source.scala
index 7d9b009c0d..afb225a3d1 100644
--- a/akka-stream/src/main/scala/akka/stream/scaladsl/Source.scala
+++ b/akka-stream/src/main/scala/akka/stream/scaladsl/Source.scala
@@ -116,7 +116,7 @@ object Source {
    * The stream ends normally when evaluation of the closure returns a `None`.
    * The stream ends exceptionally when an exception is thrown from the closure.
    */
-  def apply[T](f: () ⇒ Option[T]): Source[T] = ThunkSource(f)
+  def apply[T](f: () ⇒ Option[T]): Source[T] = IteratorSource(new ThunkIterator(f))
 
   /**
    * Start a new `Source` from the given `Future`. The stream will consist of