es5.6.3通过脚本更新时，params参数无效，更新的值变为null

{
"lang": "painless",
"script": {
"inline": "ctx._source.project_name = params.project_name"
},
"params": {
"project_name": "es"
}
}
更新之后变为：
{
"_index": "myscript",
"_type": "myscript",
"_id": "2",
"_version": 31,
"found": true,
"_source": {
"id": 2,
"count": [
1,
2,
3,
4,
5
],
"project_name": null,
}
}
但是采用硬编码是可以更新的：
{
"lang": "painless",
"script": {
"inline": "ctx._source.project_name = 'es'"
},
"params": {
"project_name": "es"
}
}