Kryo Bug?

KryoBug.scala

import java.nio.file.{Files, Paths}

import com.esotericsoftware.kryo.io.{Input, Output}
import com.twitter.chill.ScalaKryoInstantiator

import scala.collection.mutable
import scala.reflect.ClassTag

object ScoptBug extends App {
  val kryo = (new ScalaKryoInstantiator).setRegistrationRequired(false).newKryo()


  // save the map

  val myMap = mutable.HashMap[String, Set[String]]().withDefaultValue(Set())
  myMap.update("foo", Set("bar"))

  println(myMap.getClass)
  println(myMap("squirrel"))
  println()

  val cacheFile = Paths.get(
    System.getProperty("java.io.tmpdir"),
    s"${this.getClass.getSimpleName}-CachedLoader-${myMap.hashCode.toHexString}.chill")

  val os = Files.newOutputStream(cacheFile)
  try {
    kryo.writeObject(new Output(os), myMap)
  } finally {
    os.close()
  }


  // load the map

  val is = Files.newInputStream(cacheFile)
  val result = try {
    kryo.readObject(new Input(is), myMap.getClass)
  } finally {
    is.close()
  }

  println(result.getClass)
  println(result("squirrel"))
  println()


  // try to cast it

  val withDefaultMap = result.asInstanceOf[scala.collection.mutable.Map.WithDefault[String, Set[String]]]
  println(withDefaultMap.getClass)
  println(withDefaultMap("squirrel"))
  println()

  Files.deleteIfExists(cacheFile)
}

Line 44 throws an exception.

Line 50 throws an exception too, if you let it get there.

Full output:

class scala.collection.mutable.Map$WithDefault
Set()

class scala.collection.mutable.HashMap
Exception in thread "main" java.util.NoSuchElementException: key not found: squirrel
    at scala.collection.MapLike$class.default(MapLike.scala:228)
    at scala.collection.AbstractMap.default(Map.scala:59)
    at scala.collection.mutable.HashMap.apply(HashMap.scala:65)
    at ScoptBug$.delayedEndpoint$ScoptBug$1(ScoptBug.scala:39)
    at ScoptBug$delayedInit$body.apply(ScoptBug.scala:10)
    at scala.Function0$class.apply$mcV$sp(Function0.scala:40)
    at scala.runtime.AbstractFunction0.apply$mcV$sp(AbstractFunction0.scala:12)
    at scala.App$$anonfun$main$1.apply(App.scala:76)
    at scala.App$$anonfun$main$1.apply(App.scala:76)
    at scala.collection.immutable.List.foreach(List.scala:381)
    at scala.collection.generic.TraversableForwarder$class.foreach(TraversableForwarder.scala:35)
    at scala.App$class.main(App.scala:76)
    at ScoptBug$.main(ScoptBug.scala:10)
    at ScoptBug.main(ScoptBug.scala)