Akka-stream UnsupportedOperationException by creating a Source from Graph - scala

Question

I am trying to connect a stream with a n * subFlows. Therefore I build a source from the outlet of a broadcast. But it throws an UnsupportedOperationException: cannot replace the shape of the EmptyModule. I tried to google this exception, but I wasn't able to find anything similar.
Here my Code
val aggFlow = Flow.fromGraph(GraphDSL.create() { implicit builder =>
val broadcast = builder.add(Broadcast[MonitoringMetricEvent](2))
val bc = builder.add(Broadcast[Long](1))
val zip = builder.add(ZipWith[StreamMeasurement, Long, (StreamMeasurement, Long)]((value, ewma) => (value, ewma)))
val merge = builder.add(Merge[Seq[StreamMeasurement]](1))
broadcast.out(1) ~> identityFlow ~> maxFlow ~> bc
val source = Source.fromGraph(GraphDSL.create() { implicit bl =>
SourceShape(bc.out(0))
})
broadcast.out(0) ~> identityFlow ~> topicFlow.groupBy(MAX_SUB_STREAMS, _._1)
.map(_._2)
.zip[Long](source)
.takeWhile(deciderFunction)
.map(_._1)
.fold[Seq[StreamMeasurement]](Seq.empty[StreamMeasurement])((seq, sm) => seq:+sm)
.mergeSubstreams ~> merge
FlowShape(broadcast.in, merge.out)
})
and here the Exception I get:
Exception in thread "main" java.lang.ExceptionInInitializerError
at xxx$.main(Processor.scala:80)
at xxx.Processor.main(Processor.scala)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:498)
at com.intellij.rt.execution.application.AppMain.main(AppMain.java:144)
Caused by: java.lang.UnsupportedOperationException: cannot replace the shape of the EmptyModule
at akka.stream.impl.StreamLayout$EmptyModule$.replaceShape(StreamLayout.scala:322)
at akka.stream.scaladsl.GraphApply$class.create(GraphApply.scala:18)
at akka.stream.scaladsl.GraphDSL$.create(Graph.scala:801)
at xxx.logic$$anonfun$22.apply(logic.scala:156)
at xxx.logic$$anonfun$22.apply(logic.scala:146)
at akka.stream.scaladsl.GraphApply$class.create(GraphApply.scala:17)
at akka.stream.scaladsl.GraphDSL$.create(Graph.scala:801)
at xxx.logic$.<init>(logic.scala:146)
at xxx.logic$.<clinit>(logic.scala)
... 7 more
the key problem can be found here: akka-stream Zipping Flows with SubFlows