You want to update your submodule so it is in sync with what the parent repository believes it should be. This is what the update command is for:
From the submodule manpage:
Update the registered submodules, i.e. clone missing submodules and checkout the commit specified in the index of the containing repository. This will make the submodules HEAD be detached unless --rebase or --merge is specified or the key submodule.$name.update is set to rebase or merge.
Run this and all should be well:
git submodule update --init
You can add the --recursive flag as well to recurse through all submodules.