ReferenceError: sass is not defined
<p>When I configured gulp and running task for scss, I am getting Error saying   <em>ReferenceError: sass is not defined</em> even I have installed gulp-scss and declared in gulp.js</p>

<pre><code>var gulp = require('gulp');
var scss = require('gulp-sass');

gulp.task('sass', function () {
  return gulp.src('app/scss/mystyles.scss')
    .pipe(sass().on('error', sass.logError))
    .pipe(gulp.dest('app/css'));
});

gulp.task('default', ['sass']);
</code></pre>

<p>Please suggest me what I am doing wrong.</p>