Using JIRA module how to move a issue as subtask in Python

I am using below code:

from jira import JIRA
import sys

options = {'server': 'https://jira.com', 'verify': False}
jira = JIRA(options, basic_auth=('user@example.com', 'Password123'))

child_key = sys.argv[1]
parent_key = sys.argv[2]

issue = jira.issue(child_key)
update_issue = issue.update(issuetype={'name': 'Sub Task'}, parent={'key': parent_key})

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But when I run it as

python mycode.py PROJECT-123 PROJECT-100

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then I get the below error:

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

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Though I am providing the parent key, I would like to know how to fix it?