diff --git a/akka-http-core/src/main/scala/akka/http/impl/engine/server/HttpServerBluePrint.scala b/akka-http-core/src/main/scala/akka/http/impl/engine/server/HttpServerBluePrint.scala
index 9f2e56d5cc..25b29accc8 100644
--- a/akka-http-core/src/main/scala/akka/http/impl/engine/server/HttpServerBluePrint.scala
+++ b/akka-http-core/src/main/scala/akka/http/impl/engine/server/HttpServerBluePrint.scala
@@ -345,6 +345,7 @@ private[http] object HttpServerBluePrint {
   class WebsocketMerge(installHandler: Flow[FrameEvent, FrameEvent, Any] ⇒ Unit) extends FlexiMerge[ByteString, FanInShape2[ResponseRenderingOutput, ByteString, ByteString]](new FanInShape2("websocketMerge"), OperationAttributes.name("websocketMerge")) {
     def createMergeLogic(s: FanInShape2[ResponseRenderingOutput, ByteString, ByteString]): MergeLogic[ByteString] =
       new MergeLogic[ByteString] {
+        var websocketHandlerWasInstalled: Boolean = false
         def httpIn = s.in0
         def wsIn = s.in1
 
@@ -358,23 +359,20 @@ private[http] object HttpServerBluePrint {
               ctx.emit(responseBytes)
               installHandler(handlerFlow)
               ctx.changeCompletionHandling(defaultCompletionHandling)
+              websocketHandlerWasInstalled = true
               websocket
           }
         }
 
-        // Completion handling that installs a dummy handler, to make sure no processors leak because they have
-        // never been subscribed to, see #17494.
-        override def initialCompletionHandling: CompletionHandling =
-          CompletionHandling(
-            onUpstreamFinish = { (ctx, in) ⇒ installDummyHandler(); defaultCompletionHandling.onUpstreamFinish(ctx, in) },
-            onUpstreamFailure = { (ctx, in, t) ⇒ installDummyHandler(); defaultCompletionHandling.onUpstreamFailure(ctx, in, t) })
-
-        def installDummyHandler(): Unit = installHandler(Flow[FrameEvent])
-
         def websocket: State[_] = State[ByteString](Read(wsIn)) { (ctx, in, bytes) ⇒
           ctx.emit(bytes)
           SameState
         }
+
+        override def postStop(): Unit = if (!websocketHandlerWasInstalled) installDummyHandler()
+        // Install a dummy handler to make sure no processors leak because they have
+        // never been subscribed to, see #17494 and #17551.
+        def installDummyHandler(): Unit = installHandler(Flow[FrameEvent])
       }
   }
   /** A merge for two streams that just forwards all elements and closes the connection when the first input closes. */