Nevermind, I misinterpreted. I just realized it's supposed to be "viewer=[ID]", I was interpreting it as "viewer=[ID]&userid=[ID]". I'm still getting errors,

Nevermind, I misinterpreted. I just realized it's supposed to be "viewer=[ID]", I was interpreting it as "viewer=[ID]&userid=[ID]". I'm still getting errors, but presumably that's another matter.