mix smoothie.init: ** (ArgumentError) all arguments for System.cmd/3 must be binaries

[drernie]

I am trying to use smoothie with "phoenix 1.5.4". After mix deps.get, I get an error on "mix smoothie.init"

% mix smoothie.init ** (ArgumentError) all arguments for System.cmd/3 must be binaries (elixir 1.10.4) lib/system.ex:786: System.cmd/3 (mix 1.10.4) lib/mix/task.ex:330: Mix.Task.run_task/3 (mix 1.10.4) lib/mix/cli.ex:82: Mix.CLI.run_task/2 (elixir 1.10.4) lib/code.ex:926: Code.require_file/2
I assume this is from init.ex:
System.cmd "npm", ["i", @package_version, "--save-dev", @new_opts]

Has anyone else seen this? Or is there something weird about my config?

Code available here: https://github.com/TheSwanFactory/igwet/tree/smoothie