Update summary value from single list

[Florencio Morales Bermudez]

I'm making an automation, to update the summary field from a single list, but I just keep getting 

Operation value must be a string (summary)

The json that I'm using is this:

{
  "fields": {
    "summary": {
      "set": "{{customfieldname}}"
    }
  }
}

[Alex Koxaras -Relational-]

Hi @Florencio Morales Bermudez 

Try

{
"fields": {
"summary": "{{issue.customfield_XXXXX}}"
}
}