Calling doc().score() in a native script causes a NullPointerException

error.json

{
  "error" : "SearchPhaseExecutionException[Failed to execute phase [query_fetch], total failure; shardFailures {NullPointerException[null]}]",
  "status" : 500
}

query.json

{
"fields" : ["user","message"],
 "query" : {
        "text" : { "message" : "elastic" }
    },
    "script_fields": {
        "DynamicBoost": {
            "script": "DynamicBoost",
            "lang": "native",
            "params": {
                "field": "postDate"
            }
        }
    }
}