Reindex unable to parse custom mapping defined in Studio

An error occurred while putting the mapping: 
{
  "dc:subjects" : {
    "type" : "keyword",
    "copy_to" : "all_field"
  }
} 
into ElasticSearch configuration, org.elasticsearch.client.ResponseException: method [PUT], host [http://localhost:9200], URI [/nuxeo/_mapping], status line [HTTP/1.1 400 Bad Request]
{"error":{"root_cause":[{"type":"mapper_parsing_exception","reason":"Root mapping definition has unsupported parameters:  [dc:subjects : {copy_to=all_field, type=keyword}]"}],"type":"mapper_parsing_exception","reason":"Root mapping definition has unsupported parameters:  [dc:subjects : {copy_to=all_field, type=keyword}]"},"status":400}

{
  "properties": {
    "dc:subjects" : {
      "type" : "keyword",
      "copy_to" : "all_field"
    }
  }
}