diff --git a/akka-stream-tests/src/test/scala/akka/stream/scaladsl/GraphMatValueSpec.scala b/akka-stream-tests/src/test/scala/akka/stream/scaladsl/GraphMatValueSpec.scala
index cb524e0a16f..06033023610 100644
--- a/akka-stream-tests/src/test/scala/akka/stream/scaladsl/GraphMatValueSpec.scala
+++ b/akka-stream-tests/src/test/scala/akka/stream/scaladsl/GraphMatValueSpec.scala
@@ -7,8 +7,7 @@ import akka.NotUsed
 import akka.stream._
 import akka.stream.testkit._
 
-import scala.concurrent.Await
-import scala.concurrent.Future
+import scala.concurrent.{Await, Future, Promise}
 import scala.concurrent.duration._
 
 class GraphMatValueSpec extends StreamSpec {
@@ -227,5 +226,19 @@ class GraphMatValueSpec extends StreamSpec {
       matValue should ===(NotUsed)
 
     }
+
+    "not ignore materialized value of indentity flow which is optimized away" in {
+      implicit val mat = ActorMaterializer(ActorMaterializerSettings(system).withAutoFusing(false))
+      val (m1, m2) = Source.single(1).viaMat(Flow[Int])(Keep.both).to(Sink.ignore).run()
+      m1 should ===(NotUsed)
+      m2 should ===(NotUsed)
+
+      // Fails with ClassCastException if value is wrong
+      val m3: Promise[Option[Int]] = Source.maybe[Int].viaMat(Flow[Int])(Keep.left).to(Sink.ignore).run()
+      m3.success(None)
+
+      val m4 = Source.single(1).viaMat(Flow[Int])(Keep.right).to(Sink.ignore).run()
+      m4 should ===(NotUsed)
+    }
   }
 }
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 1922c058dc3..f235773ecd7 100644
--- a/akka-stream/src/main/scala/akka/stream/scaladsl/Source.scala
+++ b/akka-stream/src/main/scala/akka/stream/scaladsl/Source.scala
@@ -43,8 +43,14 @@ final class Source[+Out, +Mat](override val module: Module)
   override def via[T, Mat2](flow: Graph[FlowShape[Out, T], Mat2]): Repr[T] = viaMat(flow)(Keep.left)
 
   override def viaMat[T, Mat2, Mat3](flow: Graph[FlowShape[Out, T], Mat2])(combine: (Mat, Mat2) ⇒ Mat3): Source[T, Mat3] = {
-    if (flow.module eq GraphStages.Identity.module) this.asInstanceOf[Source[T, Mat3]]
-    else {
+    if (flow.module eq GraphStages.Identity.module) {
+      if (combine eq Keep.left)
+        this.asInstanceOf[Source[T, Mat3]]
+      else if (combine eq Keep.right)
+        this.mapMaterializedValue((_) ⇒ NotUsed).asInstanceOf[Source[T, Mat3]]
+      else
+        this.mapMaterializedValue(combine(_, NotUsed.asInstanceOf[Mat2])).asInstanceOf[Source[T, Mat3]]
+    } else {
       val flowCopy = flow.module.carbonCopy
       new Source(
         module