Failing lesslint example

Gruntfiles.js

  csslint: {
      options: {
          formatters: [
              {id: 'lint-xml', dest: 'build/reports/csslint/csslint.xml'}
          ]
      },
      strict: {
          options: {
              // IDs until re-use needed. Re-factor scaredy cats!
              ids: false,
              // Yes, never import css. Collapse it.
              import: 2,
              // who cares about IE 6 and 7?
              "box-sizing": false,
              // Assuming we know the box model...
              "box-model": false,
              // Pretty rather than accessible, I guess
              "outline-none": false,
              // This OSCSS principal isn't very DRY
              "qualified-headings": false,
              "unique-headings": false,
              // Yeah it's slow. It's also spec
              "universal-selector": false
          }
      }
  },
  less: {
      options: {
          yuicompress: false
      },
      layout: {
          files: {
            'layout.min.css' : 'layout.less'
          }
      }
  },
  lesslint: {
      src: ['*.less']
  },

layout.less

@import-once "mixin";

div.plan {
  .highlights {
    .clear-fix();
  }
}

mixin.less

.clear-fix() {
  zoom: 1; /* For IE 6/7 (trigger hasLayout) */

  &:after {
    content: ".";
    display: block;
    clear: both;
    visibility: hidden;
    line-height: 0;
    height: 0;
    clear: both;
  }
}

package.json

{
    "name": "test-lesslint",
    "version": "0.1.0",
    "devDependencies": {
        "grunt": "~0.4.1",
        "grunt-karma": "~0.4.3",
        "grunt-contrib-concat": "~0.2.0",
        "grunt-contrib-csslint": "~0.1.2",
        "grunt-contrib-jshint": "~0.6.0",
        "grunt-contrib-less": "~0.5.1",
        "grunt-lesslint": "~0.7.0",
        "grunt-contrib-uglify": "~0.2.0",
        "grunt-contrib-watch": "~0.3.1",
        "grunt-contrib-copy": "~0.4.1",
        "grunt-contrib-clean": "~0.4.1",
        "grunt-html2js": "~0.1.3"
    },
    "dependencies": {}
}