Heroku db:pull 'db:pull is not a heroku command'

Heroku db:pull 'db:pull is not a heroku command'

I'm getting this message even though I've used heroku db:pull a million
times. For some reason it's no longer working even though I haven't even
touched my code. Any ideas?
The full error message is
db:pull is not a heroku command. Perhaps you meant pg:pull See heroku help
for a list of available commands.