Inner flow was completed without producing result elements for 1 outstanding elements

[scape7yu]

I followed these steps, but error comes

$ git clone https://github.com/sangria-graphql/sangria-playground.git
$ cd sangria-playground
$ sbt run

vist http://localhost:9000/

but

akka.http.impl.util.One2OneBidiFlow$OutputTruncationException: Inner flow was completed without producing result elements for 1 outstanding elements
        at akka.http.impl.util.One2OneBidiFlow$OutputTruncationException$.apply(One2OneBidiFlow.scala:22)
        at akka.http.impl.util.One2OneBidiFlow$OutputTruncationException$.apply(One2OneBidiFlow.scala:22)
        at akka.http.impl.util.One2OneBidiFlow$One2OneBidi$$anon$1$$anon$4.onUpstreamFinish(One2OneBidiFlow.scala:97)
        at akka.stream.impl.fusing.GraphInterpreter.processEvent(GraphInterpreter.scala:523)
        at akka.stream.impl.fusing.GraphInterpreter.execute(GraphInterpreter.scala:390)
        at akka.stream.impl.fusing.GraphInterpreterShell.runBatch(ActorGraphInterpreter.scala:625)
        at akka.stream.impl.fusing.GraphInterpreterShell$AsyncInput.execute(ActorGraphInterpreter.scala:502)
        at akka.stream.impl.fusing.GraphInterpreterShell.processEvent(ActorGraphInterpreter.scala:600)
        at akka.stream.impl.fusing.ActorGraphInterpreter.akka$stream$impl$fusing$ActorGraphInterpreter$$processEvent(ActorGraphInterpreter.scala:769)
        at akka.stream.impl.fusing.ActorGraphInterpreter$$anonfun$receive$1.applyOrElse(ActorGraphInterpreter.scala:784)

it's strange