ElasticSearch update script having problems with fields starting with @ sign

gistfile1.txt

Version: 0.19.9

curl -XGET 'http://xxx:9200/_cluster/health?pretty=true'
{
  "cluster_name" : "elasticsearch",
  "status" : "yellow",
  "timed_out" : false,
  "number_of_nodes" : 2,
  "number_of_data_nodes" : 1,
  "active_primary_shards" : 85,
  "active_shards" : 85,
  "relocating_shards" : 0,
  "initializing_shards" : 0,
  "unassigned_shards" : 85
}



Works:
curl -XPUT xxx:9200/test/test/1 -d '{"counter" : 1, "tags" : ["fubar1"]}'
curl -XPOST 'xxx:9200/test/test/1/_update' -d '{"script" : "ctx._source.tags += tag","params": {"tag":"fubar2"}}'

Does not work:
curl -XPUT xxx:9200/test/test/2 -d '{"counter" : 1, "@tags" : ["fubar1"]}'
curl -XPOST 'xxx:9200/test/test/2/_update' -d '{"script" : "ctx._source.@tags += tag","params": {"tag":"fubar2"}}'

{"error":"ElasticSearchIllegalArgumentException[failed to execute script]; nested: ArrayIndexOutOfBoundsException[0]; ","status":400}