SourceMaps wrong behavior on optimize: "uglify2"

dist: {
        // Options:
        // https://github.com/jrburke/r.js/blob/master/build/example.build.js
        options: {
          name: 'main',
          mainConfigFile: '<%= paths.app %>/main.js',
          baseUrl: '<%= paths.app %>',
          optimize: 'uglify2',
          out: '<%= paths.dist %>/scripts/build.js',

          generateSourceMaps: true,
          preserveLicenseComments: false,
          skipDirOptimize: true,
          useStrict: true,
          stubModules: ['text', 'json', 'has', 'jsx', 'JSXTransformer'],
          findNestedDependencies: true,
          has: {
            production: false,
            inside: false
          },
          // wrap: true,
          deps: ['../bower_components/requirejs/require']
          //uglify2: {} // https://github.com/mishoo/UglifyJS2
        }
      }