Sourcemap output with grunt sass

Question

In my project I finally got rid of compass and using autoprefixer instead. I needed a new css compiler so I used the default grunt-contrib-sass module. These are my settings:

    sass: {
        dist: {
          files: [{
            expand: true,
            lineNumbers: true,
            cwd: 'styles',
            src: 'src/sass/app.scss',
            dest: 'src/css/app.css',
            ext: '.css'
          }]
        }
      },

But it only outputs a sourcemap url. The paths to the files are correct. In app.scss I import all my sass files / modules.