title: Unable to write "undefined" file in grunt plugin
body: <p>I'm new to writing grunt plugins and I'm running into a problem when I'm trying to run it:</p>
  
  <pre><code>Running "inject_css:dev" (inject_css) task
  Warning: Unable to write "undefined" file (Error code: undefined). Use --force to continue.
  </code></pre>
  
  <p>My plugin looks like:</p>
  
  <pre><code>'use strict';
  
  module.exports = function(grunt) {
    grunt.registerMultiTask('inject_css', 'Allows you to inject CSS into HTML files as part of the build process.', function() {
      var src = grunt.file.expand(this.data.src);
      var text = '';
  
      if (src) {
          src.forEach(function (script) {
              text += grunt.file.read(script);
          });
      } else {
          grunt.log.error('Please specify a file to inject into the html.');
          return;
      }
  
      this.files.forEach(function (file) {
          grunt.file.write(file.dest, grunt.file.read(file.src).replace('&lt;!-- inject --&gt;', '&lt;style type="text/css"&gt;' + text + '&lt;/style&gt;'));
          grunt.log.ok('File injected'.blue + ' into ' + file.dest);
      });
    });
  };
  </code></pre>
  
  <p>And when trying to call it in my gruntfile I'm using the following config:</p>
  
  <pre><code>inject_css: {
    dev: {
      files:{
        'static/test/test.html': 'test.html'
      },
      src: 'static/less/form.less'
    }
  }
  </code></pre>
  
  <p>Any ideas what I'm doing wrong?</p>