What is causing 404s when merging a branch using PyGithub?

Question

I’m trying to (fast-forward) merge a branch using PyGithub in a test-bed repository, however it always returns a 404:

#!/usr/bin/env python

import os

from github import Github
from github.GithubException import GithubException

github = Github(os.environ["GH_TOKEN"])
repo = github.get_repo(os.environ["GITHUB_REPOSITORY"])

# branches = [b.name for b in repo.get_branches()]
branches = list(repo.get_branches())
print(f"BRANCHES: {branches}")

# Merge merge-me into main
# repo.merge(base=branches[0], head=branches[1])
try:
    print("=================== BY NAME")
    repo.merge(base=branches[0].name, head=branches[1].name)
except GithubException as ge:
    print(ge)

try:
    print("=================== BY SHA")
    repo.merge(base=branches[0].name, head=branches[1].commit.sha)
except GithubException as ge:
    print(ge)

I added the SHA attempt based on this answer. However, the script produces the following output:

BRANCHES: [Branch(name="main"), Branch(name="merge-me")]
=================== BY NAME
404 {"message": "Not Found", "documentation_url": "https://docs.github.com/rest/reference/repos#merge-a-branch"}
=================== BY SHA
404 {"message": "Not Found", "documentation_url": "https://docs.github.com/rest/reference/repos#merge-a-branch"}

According to the API docs, a 404 occurs when either the base or head do not exist. But I’m pulling both from the list of (the only) branches in the repository.

I would expect a permission problem to return a 403, but I’m at a loss to explain what is going on.

Edit 1:

I was experiencing this error with a private repository in an organization with SSO. I created a public repository with a similar setup, and it ran without issue.

This token is authorized for SSO to the organization, otherwise it would not be able to access the repository at all. So it shouldn’t be causing this problem, but that is my current best guess as to what is happening.

Edit 2:

The production version of this code was accidentally released to integration and it’s working within a Github Action using the same API Token. So now my best guess is there is something in my own environment getting in my way.