Can we use _score value in script_fields in elasticsearch 6.2?

[Suyog_Kale]

Using: Elasticsearch 6.2
Default script: painless
Can we use _score value in script_fields
Below query always returns 0 value for custom_score
{
"query": {
"match_all": {}
},
"script_fields": {
"custom_score": {
"script": {
"source": "params.factor * _score",
"params": {
"factor": 200
}
}
}
}
}