branch{u} not working even though upstream configured

What could be a reason why branch@{u} doesn't work even though the branch does have a remote tracking branch configured?

We have a user who reports the following:

$ git push --set-upstream origin test-456
branch 'test-456' set up to track 'origin/test-456'.
Everything up-to-date
$ git merge-base test-456 test-456@{u}
fatal: upstream branch 'refs/heads/test-456' not stored as a
       remote-tracking branch

Git remote shows that the remote tracking branch is configured as expected:

$ git remote -v show origin
* remote origin
  [...]
  Local branches configured for 'git pull':
    test-456          merges with remote test-456
  Local refs configured for 'git push':
    test-456          pushes to test-456           (up to date)

Any ideas?