Can't get text on a START_OBJECT at 1:1111 when using the _reindex API

[Norman_Khine]

Hello,
I am using the _reindex API to merge daily indexes into monthly indexes but get this error:

{
"index": "sentry-2019.01",
"type": "_doc",
"id": "NtBfKWgBAZO4MSmwwzvg",
"cause": {
"type": "mapper_parsing_exception",
"reason": "failed to parse field [capturedMessage.data.body] of type [text] in document with id 'NtBfKWgBAZO4MSmwwzvg'",
"caused_by": {
"type": "illegal_state_exception",
"reason": "Can't get text on a START_OBJECT at 1:1111"
}
},
"status": 400
}

There is this thread Help on: Can't get text on a START_OBJECT which suggests to modify the template, but what I don't understand is that if the document was indexed and I am just reindex it, it shouldn't fail.

I am using:

POST _reindex?requests_per_second=115&wait_for_completion=true
{
  "source": {
    "index": "sentry-2019.01.*",
    "size": 1000
  },
  "dest": {
    "index": "sentry-2019.01"
  },
  "conflicts": "proceed", 
  "script": {
    "lang": "painless",   
    "source": """
      ctx._source.index = ctx._index;
    """
  }
}

Any advice is much appreciated