Getting ClassCastException[org.elasticsearch.search.controller.ShardScoreDoc cannot be cast to org.apache.lucene.search.FieldDoc]

gistfile1.txt

The "permission" item type exists on the system and library indexes. It does not exist on the 'kk' index.

curl -XPOST "http://localhost:9200/system,library,kk/permission/_search" -d {
    "from": 0,
    "size": "20",
    "sort": [
        {
            "title": {
                "ignore_unmapped": true
            }
        }
    ]
}

{"error":"ReduceSearchPhaseException[Failed to execute phase [fetch], [reduce] ]; nested: ClassCastException[org.elasticsearch.search.controller.ShardScoreDoc cannot be cast to org.apache.lucene.search.FieldDoc]; ","status":500}

Works fine when the sort is removed from the query.