Can't run powerline-config during startup (in .tmux.conf)

When I start tmux, I get a failure when trying to configure powerline. I have set an environment environment variable with this:

export POWERLINE_CONFIG_COMMAND=`which powerline-config`

My ~/.tmux.conf contains the following:

if-shell "POWERLINE_CONFIG_COMMAND" \
  run-shell "$POWERLINE_CONFIG_COMMAND tmux setup"

The error I get is:

unknown command: /path/to/powerline-config

I can run the config command manually after tmux starts with this:

$POWERLINE_CONFIG_COMMAND tmux setup

I don't understand why tmux can't run the command during the startup when it can run just fine afterwards.