_routing error with child documents

server error.log

org.elasticsearch.index.mapper.MapperParsingException: External routing [p1] and document path routing [1] mismatch
        at org.elasticsearch.index.mapper.internal.RoutingFieldMapper.validate(RoutingFieldMapper.java:176)
        at org.elasticsearch.index.mapper.DocumentMapper.parse(DocumentMapper.java:485)
        at org.elasticsearch.index.mapper.DocumentMapper.parse(DocumentMapper.java:417)
        at org.elasticsearch.index.shard.service.InternalIndexShard.prepareIndex(InternalIndexShard.java:305)
        at org.elasticsearch.action.index.TransportIndexAction.shardOperationOnPrimary(TransportIndexAction.java:202)
        at org.elasticsearch.action.support.replication.TransportShardReplicationOperationAction$AsyncShardOperationAction.performOnPrimary(TransportShardReplicationOperationAction.java:529)
        at org.elasticsearch.action.support.replication.TransportShardReplicationOperationAction$AsyncShardOperationAction$1.run(TransportShardReplicationOperationAction.java:427)
        at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:886)
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:908)
        at java.lang.Thread.run(Thread.java:619)

test.sh

curl "http://localhost:9200/test/" -XPUT

echo
curl "http://localhost:9200/test/parent/_mapping" -XPUT -d '
{
    "parent" : {
      "_routing" : {
        "required" : true,
        "path" : "ixAccount"
      },
      "properties" : {
        "ixAccount" : {
          "type" : "integer"
        },
        "parentData" : {
          "type" : "string"
        }
      }
    }
}
'

echo
curl "http://localhost:9200/test/child/_mapping" -XPUT -d '
{
    "child" : {
      "_parent" : {
        "type" : "parent"
      },
      "_routing" : {
        "required" : true,
        "path": "ixAccount"
      },
      "properties" : {
        "ixAccount" : {
          "type" : "integer"
        },
        "childData" : {
          "type" : "string"
        }
      }
    }
}
'

echo
curl "http://localhost:9200/test/_mapping?pretty"

echo
curl "http://localhost:9200/test/parent/p1" -d '
{
    "ixAccount": 1,
    "parentData": "parent 1"
}
'

echo
curl "http://localhost:9200/test/parent/p1?pretty"

echo
# this line causes:
# {"error":"MapperParsingException[External routing [p1] and document path routing [1] mismatch]","status":400}
curl "http://localhost:9200/test/child/c1?parent=p1" -d '
{
    "ixAccount": 1,
    "child": "parent 1"
}
'

echo
curl "http://localhost:9200/test/child/c1?pretty"

echo
# but this works:
curl "http://localhost:9200/test/child/c2?parent=p1&routing=1" -d '
{
    "ixAccount": 1,
    "child": "parent 1"
}
'

echo
curl "http://localhost:9200/test/child/c2?pretty"