Child document without parent causes a NullPointerException

[srgclr]

Posting a child document without a parent causes a search exception.

Elasticsearch version: 5.1.2
Plugins installed: []
JVM version: 1.8.0_45
OS version: Windows Server 2012

Steps to reproduce:
1.

PUT my_index
{
  "mappings": {
    "my_parent": {},
    "my_child": {
      "_parent": {
        "type": "my_parent" 
      }
    }
  }
}

PUT my_index/my_child/2?routing=something
{
  "text": "This is a child document"
}

GET my_index/my_child/_search

Results:

{
  "took": 1,
  "timed_out": false,
  "_shards": {
    "total": 5,
    "successful": 4,
    "failed": 1,
    "failures": [
      {
        "shard": 2,
        "index": "my_index",
        "node": "wHB0VLvxS1Oot6IJaVU3Jg",
        "reason": {
          "type": "null_pointer_exception",
          "reason": null
        }
      }
    ]
  },
  "hits": {
    "total": 1,
    "max_score": 1,
    "hits": []
  }
}

Provide logs (if relevant):

[2017-01-24T16:36:35,858][DEBUG][o.e.a.s.TransportSearchAction] [XXX_coordinator] [574] Failed to execute fetch phase
org.elasticsearch.transport.RemoteTransportException: [XXX_data][192.168.10.154:64935][indices:data/read/search[phase/fetch/id]]
Caused by: java.lang.NullPointerException
	at org.elasticsearch.search.fetch.subphase.ParentFieldSubFetchPhase.getParentId(ParentFieldSubFetchPhase.java:66) ~[elasticsearch-5.1.2.jar:5.1.2]
	at org.elasticsearch.search.fetch.subphase.ParentFieldSubFetchPhase.hitExecute(ParentFieldSubFetchPhase.java:49) ~[elasticsearch-5.1.2.jar:5.1.2]
	at org.elasticsearch.search.fetch.FetchPhase.execute(FetchPhase.java:163) ~[elasticsearch-5.1.2.jar:5.1.2]
	at org.elasticsearch.search.SearchService.executeFetchPhase(SearchService.java:502) ~[elasticsearch-5.1.2.jar:5.1.2]
	at org.elasticsearch.action.search.SearchTransportService$13.messageReceived(SearchTransportService.java:356) ~[elasticsearch-5.1.2.jar:5.1.2]
	at org.elasticsearch.action.search.SearchTransportService$13.messageReceived(SearchTransportService.java:353) ~[elasticsearch-5.1.2.jar:5.1.2]
	at org.elasticsearch.transport.RequestHandlerRegistry.processMessageReceived(RequestHandlerRegistry.java:69) ~[elasticsearch-5.1.2.jar:5.1.2]
	at org.elasticsearch.transport.TcpTransport$RequestHandler.doRun(TcpTransport.java:1385) ~[elasticsearch-5.1.2.jar:5.1.2]
	at org.elasticsearch.common.util.concurrent.ThreadContext$ContextPreservingAbstractRunnable.doRun(ThreadContext.java:527) ~[elasticsearch-5.1.2.jar:5.1.2]
	at org.elasticsearch.common.util.concurrent.AbstractRunnable.run(AbstractRunnable.java:37) ~[elasticsearch-5.1.2.jar:5.1.2]
	at java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source) ~[?:1.8.0_45]
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source) ~[?:1.8.0_45]
	at java.lang.Thread.run(Unknown Source) [?:1.8.0_45]