Summing Story Points from story subtasks

[Babi Teixeira]

Hello! 

I'm triyng sum  story points of subtasks and replaced total value on Story points (in its parent), on the NEXT GENERATIONS version of JIRA.

But, my rules return this error: "Error while parsing additional fields. Not valid JSON."

Here is my Json:

{
  "fields": {
    "Story point estimate": {{issue.subtasks.Story point estimate.join(" +         ").asJsonNumber}}
  }
}

When i add quotes, the error returned is "Fields is not a number". 

When I change the value to a generic number, the rule run without errors.

Someone can help-me to mount this JSON?  

[Jira Guru]

{
   "fields": {
      "Story point estimate": {{#=}}{{subtasks.Story point estimate.join(" + ")}}{{/}}

   }
}