"git push" says "src refspec XYZ matches more than one" even without explicit XYZ argument.

1. My repo has a branch named 'v1' that is tracking 'origin/v1'.
2. My repo has a tag named 'v1'.
3. I have "push.default" set to "upstream".

I made a commit on branch 'v1' and tried doing a push:

    # git push
    error: src refspec v1 matches more than one.
    error: failed to push some refs to 'git@github.com:whatever/ns1-go.git'

If I rename the branch to 'v1-dev', then the push goes through.

I understand why the command "git push origin/v1 v1" is ambiguous. But if I do a plain "git push", I thought Git would know to push my current branch.

[Git version 2.10.1 from Homebrew on Mac OS 10.11.6.]