Unknow funciton call while reindex with ES5.6.8

[xlc]

I'm doing with ES5.6.8 reindex from remote ES2.3.2.Here is my script
{
"source": {
"remote": {
"host": "http://track-test:9200"
},
"index": "track_report_basic_date_17"
},
"dest": {
"index": "track_report_basic_date_17"
},
"script": {
"source": "ctx._id
=UUID.nameUUIDFromBytes(Base64.Decoder.decode(ctx._id.toString()))"
}
}
and the error message:

overview
nodes
rest
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track_report_5.6.8

_reindex?pretty

previous requests

{
"error": {
"root_cause": [
{
"type": "script_exception",
"reason": "compile error",
"script_stack": [
"... DFromBytes(Base64.Decoder.decode(ctx._id.toString( ...",
" ^---- HERE"
],
"script": "ctx._id = UUID.nameUUIDFromBytes(Base64.Decoder.decode(ctx._id.toString()))",
"lang": "painless"
}
],
"type": "script_exception",
"reason": "compile error",
"script_stack": [
"... DFromBytes(Base64.Decoder.decode(ctx._id.toString( ...",
" ^---- HERE"
],
"script": "ctx._id = UUID.nameUUIDFromBytes(Base64.Decoder.decode(ctx._id.toString()))",
"lang": "painless",
"caused_by": {
"type": "illegal_argument_exception",
"reason": "Unknown call [decode] with [1] arguments on type [Base64.Decoder]."
}
},
"status": 500
}
As the _id in ES2.3.2 longer than 512,reindex had failed.I want to generate uuid to replace old _id.Anyone can help?