Bash completion- Prompt printing nothing- OSX

So I was looking to add the prompt completion addon to my bash profile, but after finally being able to get it to accept __docker_machine_ps1 as a command, it prints nothing on my prompt. I’m expecting “default” or something to appear. Just so I know it’s working. What should I be seeing? There’s no information at all in the docs on this. Here’s my .bash_profile

for f in /usr/local/etc/bash_completion.d/docker*; do source $f; done

export PS1="------------------------------------------\n\W @ \h $(__docker_machine_ps1) => "

And my prompt looks like this:

~ @ Me  =>

Any ideas?