Is Stream#addString correct?

1_.scala

scala> Iterator.iterate(0)(_ + 1).take(10).addString(new StringBuilder, ",")
res7: StringBuilder = 0,1,2,3,4,5,6,7,8,9

scala> Stream.iterate(0)(_ + 1).take(10).addString(new StringBuilder, ",")
res8: StringBuilder = 0,?

scala> Iterator.iterate(0)(_ + 1).take(10).toString
res9: String = non-empty iterator

scala> Stream.iterate(0)(_ + 1).take(10).toString
res10: String = Stream(0, ?)