sourceMapRootpath to have sourcemaps using LESS

I'm trying to have sourcemaps on my project. I'm using LESS, compiled with Grunt using grunt-contrib-less

Here is the code I have on my gruntfile.js:

less: {
  development: {
    options: {
      paths: ["assets-src"],
      // LESS source maps
      // To enable, set sourceMap to true and update sourceMapRootpath
         based on your install
      sourceMap: true,
      sourceMapFilename: 'assets-src/desktop/css/desktop.css.map',
      sourceMapRootpath: 'assets-dist/desktop/css/'
    },
    files : {
      "assets-src/desktop/css/desktop.css" :
      [
      "assets-src/desktop/less/reset.less",
      "assets-src/desktop/less/variables.less",
      "assets-src/desktop/less/mixins.less" 
      ]
    }
  }
 }

And this is the file's structure I have:

I have problems defining the sourceMapRootpath. I tried putting the same folder where all .LESS files are, but nothing happends, the same using the folder where .CSS files are.

Any idea on this?

Thanks! seba