Sauce Labs proxy works but does not run Protractor tests

I am trying to integrate with Sauce Labs to run my project’s Protractor tests, using CircleCI’s guide as a basis. This is my circle.yml file:

    - sudo apt-get update; sudo apt-get install ruby-sass
    - npm install -g bower
    - bower install
    - wget
    - tar -xzf sc-latest-linux.tar.gz

    - cd sc-*-linux && ./bin/sc --user $SAUCE_USERNAME --api-key $SAUCE_ACCESS_KEY --readyfile ~/sauce_is_ready:
        background: true
    # Wait for tunnel to be ready
    - while [ ! -e ~/sauce_is_ready ]; do sleep 1; done
    - grunt test:protractor
        background: true
    - killall --wait sc  # wait for Sauce Connect to close the tunnel

The Sauce Connect proxy clearly works according to build output. But at that point, it totally stalls. No effort to call grunt test:protractor, no activity whatsoever. After 10 minutes I killed the build.

What am I missing from this configuration to run Protractor tests through the Sauce Labs proxy?