Elasticsearch5.6 : [reindex] failed to parse field [script]

在看Elasticsearch5.6的文档，看到mapping这一段（“Custom type field”）：
https://www.elastic.co/guide/e ... types
按着教程走的，但是输入代码;
POST _reindex
{
"source": {
"index": "twitter"
},
"dest": {
"index": "new_twitter"
},
"script": {
"source": """
ctx._source.type = ctx._type;
ctx._id = ctx._type + '-' + ctx._id;
ctx._type = 'doc';
"""
}
}
后，就报错了：
{
"error": {
"root_cause": [
{
"type": "parsing_exception",
"reason": "[reindex] failed to parse field [script]",
"line": 9,
"col": 5
}
],
"type": "parsing_exception",
"reason": "[reindex] failed to parse field [script]",
"line": 9,
"col": 5,
"caused_by": {
"type": "illegal_argument_exception",
"reason": "[script] unknown field [source], parser not found"
}
},
"status": 400
}
不知道是怎么回事，我的es和kibana都是5.6的。
希望有些眉目的前辈能点拨一下，谢谢。