TPM doesn't run from separate config sourced inside of .tmux.conf

I use Thoughtbot's dotfiles to manage all my dotfiles. All of the default configs from their dotfiles look for and source a local version. So at the bottom of .tmux.conf there is

# Local config
if-shell "[ -f ~/.tmux.conf.local ]" 'source ~/.tmux.conf.local'

Adding the plugins and run ~/.tmux/plugins/tpm/tpm inside of .tmux.conf.local does not work. The install says it's complete and to hit Enter but hitting Enter does nothing. The plugins don't get downloaded at all.