timestamp doesn't work with the format "fields.release_date"

[lveronique]

the timestamp works like =>tsfield="timestamp"
but doesn't work like =>tsfield="fields.release_date"
===example dataset===
{
"_index": "movies",
"_type": "movie",
"_id": "40",
"_version": 1,
"_score": 2,
"_source": {
"fields": {
"directors": [
"Joss Whedon"
],
"release_date": "2015-04-29T00:00:00Z",
"genres": [
"Action",
"Adventure",
"Fantasy",
"Sci-Fi"
],
"image_url": "......",
"title": "The Avengers: Age of Ultron",
"rank": 40,
"year": 2015,
"actors": [
"Scarlett Johansson",
"Chris Hemsworth",
"James Spader"
]
},
"id": "tt2395427",
"type": "add"
},
"fields": {
"fields.release_date": [
"2015-04-29T00:00:00.000Z"
]
}
}

External search command 'ess' returned error code 1. Script output = "error_message=KeyError at ".../bin/elasticsplunk.py", line 188 : u'fields.release_date' "