Why is my repo object's master_branch property None?

Hello,

I'm trying to programatically get the name of the master branch of my repo. To do this I'm trying to access the master_branch property in Repository but even though the documentation says it should return a string, I get back None regardless of the repo I've checked.

The repos are all private work-related repos so I can't share the exact code here but basically what I'm doing is:

>>> import github
>>> gh = github.Github("ACCESS_TOKEN_REDACTED")
>>> org = gh.get_organization("ORG_NAME_REDACTED")
>>> repo = org.get_repo("REPO_NAME_REDACTED")
>>> branch = repo.master_branch
>>> branch is None
True

Does this suggest I've configured my repo incorrectly? Or am I misunderstanding how to properly use the library?

Thanks in advance. Please let me know what other information might be helpful for troubleshooting.

Start at the Repository.master_branch entry point and compare its documented behavior with the GitHub API response for a private repository. Check the reported reproduction against the current library and API behavior; done means identifying whether the property or its usage needs correction and documenting a concrete resolution.