Can you check the logs on Elasticsearch side?
If you see something like this:
[2015-12-02 00:11:34,633][INFO ][rest.suppressed ] /.marvel-es-*/_field_stats Params: {level=indices, ignore_unavailable=true, index=.marvel-es-*}
java.lang.NullPointerException
at org.elasticsearch.action.fieldstats.TransportFieldStatsTransportAction.newResponse(TransportFieldStatsTransportAction.java:122)
I think you hit this issue: https://github.com/elastic/elasticsearch/pull/14868
It should be fixed in the next bugfix release.