Gulp gives The following tasks did not complete: sass. Did you forget to signal async completion?

Question

Node.js

(function(r){
  let gulp = r('gulp'),
    sass = r('gulp-sass');

  gulp.task('sass', function() {
    return gulp.src('./css/**/*.scss')
      .pipe(sass())
      .pipe(gulp.dest('dist/'));
  });
}(require));

Gulp gives The following tasks did not complete: sass. Did you forget to signal async completion?
Why?