Gulp-phantomcss throwing errors

I’m at a total loss. I’m trying to run gulp-phantomcss as part of my testing and i’m getting the error

Error: Cannot find module 'casper'

  phantomjs://platform/bootstrap.js:299 in require
  phantomjs://platform/bootstrap.js:263 in require

the build was canceled

Is anyone else using gulp-phantomcss successfully and able to post a snippet?

gulp.task('testing:phantomcss:mobile', ['patternlab:host'], function () {
  return gulp.src('./tests/phantomcss/**/*.js')
      "screenshots" : "./tests/phantomcss/screenshots/mobile",
      "failures" : `${ARTIFACTS_PATH}/phantomcss`,
      "viewportSize": [320, 600]