es.map file undefined

gulpfile.js

var gulp = require('gulp');
var es = require('event-stream');

function plugalug() {
  return es.map(function(file) {
    if (typeof file === 'undefined') {
      throw new Error('File was undefined!');
    }
  });
}

gulp.task('test-watch', function() {
  return gulp.src('gulpfile.js')
    .pipe(plugalug());
})

gulp.task('default', function() {
  gulp.watch('gulpfile.js', function(event) {
    gulp.run('test-watch')
  });
});