If you use it on a fresh install, with a non existant DB, you have to build it :

curl -XPUT http://YOURSERVER:9200/YOURINDEX/ -d '
{
 "settings":
 {
  "events":
  {
   "mappings":
   {
    "@source":
     {"type":"string"},
    "@source_host":
     {"type":"string"},
    "@source_path":
     {"type":"string"},
    "@timestamp":
     {"type":"date"},
    "@tags":
     {"type":"string"},
    "@message":
     {"type":"string"},
    "@fields":
    {
     "type":"object",
     "properties":
     {
      "HOSTNAME":
       {"type":"string"}
     }
    }
   }
  }
 }
}'