Painless script. Illegal list shortcut value [value]. For try to stup field variable

Hello all,

I tried with Painless script to create a new field from another field.

I have this code

if(doc['name.keyword'].size()!=0){
      def p =/[^\/]+\/([^\/]+)/;
      def m = p.matcher(doc['name.keyword'].value);
       if (m.find()) {
        doc['test_name.keyword'].value = m.group(1);
        return m.group(1);
      }
}

If i use only the return is correct is show the correct text on the kibana but i want also to be on the _doc

The error:

Caused by: java.lang.IllegalArgumentException: Illegal list shortcut value [value].
        at org.elasticsearch.painless.Def.lookupSetter(Def.java:443) ~[?:?]
        at org.elasticsearch.painless.DefBootstrap$PIC.lookup(DefBootstrap.java:156) ~[?:?]
        at org.elasticsearch.painless.DefBootstrap$PIC.fallback(DefBootstrap.java:208) ~[?:?]
        at org.elasticsearch.painless.PainlessScript$Script.execute(if(doc['name.keyword'].size()!=0){ ...:201) ~[?:?]
        at org.elasticsearch.search.fetch.subphase.ScriptFieldsFetchSubPhase.hitsExecute(ScriptFieldsFetchSubPhase.java:67) ~[elasticsearch-7.1.1.jar:7.1.1]
        at org.elasticsearch.search.fetch.FetchPhase.execute(FetchPhase.java:177) ~[elasticsearch-7.1.1.jar:7.1.1]
        at org.elasticsearch.search.SearchService.lambda$executeFetchPhase$3(SearchService.java:540) ~[elasticsearch-7.1.1.jar:7.1.1]
        at org.elasticsearch.search.SearchService$3.doRun(SearchService.java:380) ~[elasticsearch-7.1.1.jar:7.1.1]
        at org.elasticsearch.common.util.concurrent.AbstractRunnable.run(AbstractRunnable.java:37) ~[elasticsearch-7.1.1.jar:7.1.1]
        at org.elasticsearch.common.util.concurrent.TimedRunnable.doRun(TimedRunnable.java:41) ~[elasticsearch-7.1.1.jar:7.1.1]
        at org.elasticsearch.common.util.concurrent.ThreadContext$ContextPreservingAbstractRunnable.doRun(ThreadContext.java:751) ~[elasticsearch-7.1.1.jar:7.1.1]

I'm not sure if it's completely related but it seems you should be returning something in the case that the matcher does not match as well.