Update of _ttl not working with doc option

Hi I'm trying to update the ttl of a document with the following way but it seems that it is not getting updated:

POST /my_index/my_type/AU4Gd1DVbqjanfsolMgP/_update
"doc": {
"_ttl": 60000
"doc_as_upsert": true

With the script way it is getting updated normally.. What is the problem? does anyone know? thanks

related stackoverflow question: link

Does anyone have an answer to this?