.setFields("_source.obj") vs .setFields("obj"): The latter results in NullPointerException

Test_1.scala

package test

import scala.collection.JavaConversions._
import org.elasticsearch.node.NodeBuilder._
import org.elasticsearch.action.admin.indices.create.CreateIndexRequest
import org.elasticsearch.client.Requests._
import org.elasticsearch.action.admin.cluster.health.ClusterHealthRequest

object Test_1 extends App {

  val node = nodeBuilder().local(true).node
  val client = node.client
  val source = """{"message": "hi", "field": 123, "obj":{"abc": "123"}}"""

  try {
    client.admin.indices.create(new CreateIndexRequest("twitter")).actionGet()
  } catch {
    case e => println("Index already exists " + e);
  }
  client.admin.cluster.health(new ClusterHealthRequest("twitter").waitForYellowStatus).actionGet()

  val indexResponse = client.prepareIndex("twitter", "tweet")
    .setSource(source)
    .setId("id1")
    .execute()
    .actionGet()

  client.admin().indices().refresh(refreshRequest()).actionGet()

  try {
    val getResponse2 = client.prepareGet("twitter", "tweet", "id1")
      .setFields("_source.obj")
      .execute()
      .actionGet()
    println("Response " + ((getResponse2.field("_source.obj")).values()))

    val getResponse1 = client.prepareGet("twitter", "tweet", "id1")
      .setFields("obj")
      .execute()
      .actionGet()
    println("Response " + ((getResponse1.field("obj")).values()))

    node.close()
    println("--- done ---")
  } catch {
    case e => println(e)
  }
}