Script on nested fileds

hi all and sorry for my poor english .
I have an index with this mapping




        "type": "text",

        "fields":{"keyword":{"type": "keyword", "ignore_above": 256 }}



           "type": "keyword" 



            "type": "nested",


                "detail":{"type": "keyword"},

                "identifier":{"type": "keyword"},

                "modelType":{"type": "keyword"},

                "notificationDate":{    "type": "long"     } 




I'm creating a script on notificatioDate fields.
But when I do doc['notifications.notificationDate'].value , kibana says me 'Script not valid' with no reason specified

Consider that it 's searchable and aggragatable.

Here's a Stack Overflow post on using _source instead of doc in the script: elasticsearch - How to access nested property at script_fields on elastic 5.x - Stack Overflow