Elasticsearch significant terms background filtering size oddities

repro.sh

curl -XDELETE localhost:9200/test
curl -XPOST localhost:9200/test/test -d '{"a": "foo"}'
curl -XPOST localhost:9200/test/test -d '{"a": "foo"}'
curl -XPOST localhost:9200/test/test -d '{"a": "foo2"}'
curl -XPOST localhost:9200/test/test -d '{"a": "foo2"}'
curl -XPOST localhost:9200/test/test -d '{"a": "foo"}'
curl localhost:9200/test/_refresh

# ElasticsearchIllegalArgumentException[supersetFreq > supersetSize, in JLHScore.score(..)]
curl 'localhost:9200/test/_search?pretty&search_type=count' -d '{
    "aggregations": {
        "test": {
	    "significant_terms": {
		"field": "a",
		"background_filter": {
		    "match_all": {}
		}
	    }
        }
    }
}'