I can click, pull and check upstream / master without problems, but I get a warning. Upstream / master is designed to track changes made to the source code from which I forked (following github instructions).
eg.
$ git checkout upstream/master
warning: refname upstream/master is ambiguous
$ git branch -a
* master
upstream/master
remotes/origin/HEAD -> origin/master
remotes/origin/master
remotes/upstream/master
This command also has an ambiguous error:
$ git branch
* master
upstream/master
$ git checkout upstream/master filename.bar
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