Elasticsearch reindex with inline script - casting of nested values

elasticsearch reindex inline script.json

POST _reindex
{
  "source": {
    "index": "vehicles"
  },
  "dest": {
    "index": "vehicels-back"
  },
  "script": {
    "lang": "painless",
    "inline": "def fuel_types = ctx._source.fuel_types; for(ft in fuel_types){if(ft.containsKey('tank_number')) {ft.tank_number = ft.tank_number.toString()}} ctx._source.fuel_types=fuel_types"
  }
}