Git: warning: refname 'master' is ambiguous

I saw a few people asking the same question on here but it seems none of their advice is applicable to me. I'm getting the warning that is in the title of this but I don't have any tags named "master". This is the result of git branch -a:

* master
  remotes/origin/HEAD -> origin/master
  remotes/origin/master

Any idea what could be going wrong here? I've only been using git for a few months now, so it mostly just worries me that this ambiguity might mess with the repo in the future.

For me I tracked down the source of this warning to much earlier when I incorrectly issued an "update-ref" command. If you forget to specify the full refs/heads/mybranchname path in the first arg, then a file .git/mybranchname gets created, which later leads to this warning when you try to switch to that branch.

It is solved by simply deleting the .git/mybranchname, eg:

rm .git/master

And for reference, the correct form for the update-ref command is:

git update-ref refs/heads/mybranchname mytargetbranch

Don't forget the "refs/heads" part!

Also, my most common use-case for update-ref is simply manually moving a branch to point to another commit, which I've found a simpler syntax to do:

git branch -f myBranchToMove destinationBranchOrHash

This syntax is easier for me because it doesn't require that error-prone refs/heads path qualifier.

As detailed in "Git: refname 'master' is ambiguous", that means that, beside heads/master, you have another master in one of the following namespace within the git repo:

refs/<refname> 
refs/tags/<refname>
refs/heads/<refname>
refs/remotes/<refname>
refs/remotes/<refname>/HEAD

Or even ./<refname>, as mentioned in Magnus's answer.