running grunt task twice results in error Cannot call method 'forEach' of undefined Use

Question

New to grunt I have successfully used it to run purifycss. Now I want to run purifycss twice with different arguments, and ideally concat the two resulting files afterwards. What I have so far is this Gruntfile:

module.exports = function(grunt) {

  grunt.initConfig({
    purifycss: {
      bootstrap: {
        options: {},
        target: {
              src: [ 'live/*.twig' ],
              css: ['live/css/bootstrap*.css'],
              dest: 'purified_bootstrap.css'
                }
            },
      mycss: {
        options: {},
        target: {
              src: [ 'live/*.twig' ],
              css: ['live/css/mycss.css'],
              dest: 'purified_mycss.css'
                }
            }
      }
  });

  grunt.loadNpmTasks('grunt-purifycss');
  grunt.registerTask('purify', ['purifycss:bootstrap', 'purifycss:mycss']);
};

The output I get when running grunt purify from terminal:

> grunt purify
Running "purifycss:bootstrap" (purifycss) task
Warning: Cannot call method 'forEach' of undefined Use --force to continue.
Aborted due to warnings.

Any help on what's going wrong here appreciated!