ntfy doesn't work inside of tmux on OS X

I've got ntfy working beautifully in OS X on Python 3.5... when in a bare iTerm2 session. I typically work inside of tmux though, and inside of tmux, `ntfy send test` does nothing, and the shell integration does nothing. tmux is black magic to me, so I'm not sure what difference it makes, or if this issue is special to OS X, or a general problem across platforms.