git checkout tag, git pull fails in branch

I had the same problem and fixed it with this command:

$ git push -u origin master

From the help file the -u basically sets the default for pulls:

-u, --set-upstream`

  For every branch that is up to date or successfully pushed, add 
  upstream (tracking) reference, used by argument-less git-pull(1) and
  other commands. For more information, see branch.<name>.merge in 
  git-config(1).

Edit: For newer versions of Git, --set-upstream master has been deprecated, you should use --set-upstream-to instead:

git branch --set-upstream-to=origin/master master

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As it prompted, you can just run:

git branch --set-upstream master origin/master

After that, you can simply run git pull to update your code.