Purifycss not deleting unused css class

I have this code trying to delete the class hello3:

var purify = require('purify-css');
    var content = '<div class="hello"></div><div class="hello2"></div>';
    var css = '.hello { color: green; } .hello3 { display: block; }';

    var options = {
      output: 'purified.css',

      // Will minify CSS code in addition to purify.
      minify: true,

      // Logs out removed selectors.
      rejected: true
    };

    purify(content, css, options);

The output in purified.css is the same as the variable css:

.hello { color: green; } .hello3 { display: block; }

How to solve it?

via Adrian Martínez