Keep getting "Issue type is a sub-task but parent issue key or id not specified."

[Dev]

I am trying to update an issue as a sub-task. According to the documentation, the issuetype specified must be a sub-task, and I must provide the parent id or key. I have made sure I'm correctly doing both, the type is a sub-task and I'm providing the parent id 

{
"fields":
  {
    "parent":{"id":"60558"},
    "summary":"Story",
    "description":"",
    "issuetype":{"id":"5"}
  }
}

But I keep getting "Issue type is a sub-task but parent issue key or id not specified." I've also tried using the parent key to no avail. Any ideas why? For reference, here is the curl request I'm making to test this out:

curl \
-D- \
-u XXXXXX \
-X PUT \
--data '{
"fields":
{
"parent":{"id":"60558"},
"summary":"Story 14",
"description":"",
"issuetype":{"id":"5"}
}
}' \
-H "Content-Type: application/json" \
https://XXXXXXX/rest/api/2/issue/60566

According to https://community.atlassian.com/t5/Answers-Developer-Questions/Convert-task-to-subtask-via-JIRA-REST-API/qaq-p/533484 tasks can not be converted to subtasks via the REST API.

[Elijah Scherz]

I'm getting the same error and also passing in the parent key and id, but am not trying to change the issue type. Just some simple fields such as summary and description.

Did you ever end up doing anything else that allowed you to figure out how to using update() on a subtask?

[Dev]

According to https://community.atlassian.com/t5/Answers-Developer-Questions/Convert-task-to-subtask-via-JIRA-REST-API/qaq-p/533484 tasks can not be converted to subtasks via the REST API.