Oops yes, that should have been -i '' with a space. But I see now that GNU doesn't accept -i '', or even -i .orig. It MUST be without a space: "-i.orig". The reason for that I assume is there's no way to disambiguate:

  echo foo | sed -i pattern
  echo foo | sed -i .orig

But you can still use "-i.orig"; that should work on both. That will leave you with a .orig file to clean up, but arguably that's not a bad thing as -i can clobber files.