group and iterate over group values

group.scala

def groupAndGenerateNewSurrogateKey: Pipe = {
     pipe.groupBy('naturalKey){ group =>
       group.mapStream[Long, (Long, Long)]('someValueField -> ('someValueField, 'newSurrogateKey)) { items: Iterator[Long] =>
         val newSurrogateKey = KeyGen.generate()
         println(s"new group key:[$newSurrogateKey]") //outputs generated key
         println(s"items: ${items.toList}")           //correctly outputs grouped items
         items.map((_,newSurrogateKey)).toList
       }
     }.project('someValueField, 'newSurrogateKey)
   }
   //returns NOTHING... Why?