What's wrong with flow join the way i use

hello there, i am a newbie to Akka. I learn Akka Stream recently, and i see a method join within flow. I try to run an example with it. The code like following:

val f1 = Flow[Int].fold(0)(_+_)
val f2 = Flow[Int].fold(0)(_+_)
f1.join(f2)

Run it and then produce NullPointerException as following:

java.lang.NullPointerException
  akka.stream.Attributes$SourceLocation.locationName$lzycompute(Attributes.scala:319)
  akka.stream.Attributes$SourceLocation.locationName(Attributes.scala:311)
  akka.stream.Attributes$SourceLocation.toString(Attributes.scala:322)
  pprint.Walker.$anonfun$treeify$9(Walker.scala:108)
  pprint.Renderer.str$lzycompute$1(Renderer.scala:132)
  pprint.Renderer.str$1(Renderer.scala:130)
  pprint.Renderer.rec(Renderer.scala:134)
  pprint.Renderer.rec(Renderer.scala:41)
  pprint.Renderer.rec(Renderer.scala:41)
  pprint.Renderer.rec(Renderer.scala:41)
  pprint.Renderer.rec(Renderer.scala:41)
  pprint.Renderer.rec(Renderer.scala:41)
  pprint.Renderer.rec(Renderer.scala:41)
  pprint.PPrinter.tokenize(PPrinter.scala:105)
  ammonite.repl.FullReplAPI$Internal.print(FullReplAPI.scala:106)
  ammonite.repl.FullReplAPI$Internal.print$(FullReplAPI.scala:61)
  ammonite.repl.FullReplAPI$$anon$1.print(FullReplAPI.scala:34)

I search for help but no lucky. Any detail about this? please.

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