Error spawn Enoent Gruntjs on win 8.1

Question

I'm trying to run a basic app with gruntjs. Everything works fine, but once I try to run my sass-related grunt task, I have this error:

$ grunt sass
Running "sass:dist" (sass) task
Warning: spawn ENOENT Use --force to continue.

I already have installed Ruby and Sass, and Nodejs. This is my Gruntfile.js:

module.exports = function(grunt) {
grunt.initConfig({
pkg: grunt.file.readJSON('package.json'),
sass: {
  dist: {
    options: {
      style: 'expanded'
    },
    files: {
      'css/main.css': 'css/main.scss'
    }
  }
},
watch: {
  css: {
    files: ['css/main.scss'],
    tasks: ['sass']
  }
},
});
grunt.loadNpmTasks('grunt-contrib-sass');

grunt.registerTask('default', ['sass']);

};

Ruby and Sass versions:

$ ruby -v
ruby 2.0.0p598 (2014-11-13) [x64-mingw32]
$ sass -v
Sass 3.4.9 (Selective Steve)

Ruby Path is already in my PATH variable.