Doc values are unmodifiable while using scripted metric

[nmeaux]

Hello,

I am using a scripted metric, which is :

  "scripted_metric": {
"init_script": "state.nodes = new HashMap();",
"map_script": "int jtime = 1538438400 ; for (int i = jtime; i < jtime+2; ++i) {if (doc.time_start.value < i && doc.time_end.value > i) {state.nodes[i] = doc.nodelist_expand}}",
"combine_script": "def result = new HashMap(); for (entry in state.nodes.entrySet()) { if (result.containsKey(entry.getKey())) { for (n in entry.getValue()) { result[entry.getKey()].add(n); } } else { result[entry.getKey().toString()]=entry.getValue(); }} return result;",
"reduce_script": "def result = new HashMap(); for (s in states) { for (entry in s.entrySet()) { if (result.containsKey(entry.getKey())) { for (n in entry.getValue()) { result[entry.getKey()].add(n); } } else { result[entry.getKey().toString()]=entry.getValue(); } } } return result;"  }

And i get this error :

{
  "error": {
    "root_cause": [],
    "type": "search_phase_execution_exception",
    "reason": "",
    "phase": "fetch",
    "grouped": true,
    "failed_shards": [],
    "caused_by": {
      "type": "script_exception",
      "reason": "runtime error",
      "script_stack": [
        "org.elasticsearch.index.fielddata.ScriptDocValues.add(ScriptDocValues.java:97)",
        "java.util.AbstractList.add(AbstractList.java:108)",
        "result[entry.getKey()].add(n); } } else { ",
        "                      ^---- HERE"
      ],
      "script": "def result = new HashMap(); for (s in states) { for (entry in s.entrySet()) { if (result.containsKey(entry.getKey())) { for (n in entry.getValue()) { result[entry.getKey()].add(n); } } else { result[entry.getKey().toString()]=entry.getValue(); } } } return result;",
      "lang": "painless",
      "caused_by": {
        "type": "unsupported_operation_exception",
        "reason": "doc values are unmodifiable"
      }
    }
  },
  "status": 503
} 

I dont understand it, i dont try to modify a doc values, just a hashmap which is define in the reduce_script in order to make some kinds of calculation.
reduce_script & combine_script are very similar, but as combine script is working well, reduce, not.
I am using elasticsearch : 6.6.0 , could this be considered as a bug, or something i did not understand ?
thx

rgds

Nicolas