Grunt cssmin deletes every first css rule in every file

Question

Builds without misstakes, but every first css rule block are deleted. What can cause the problem?

Grunt configurations: ...

useminPrepare: {
          html: 'app/index.html',
          options: {
            dest: 'docs'
          }
        },

    // Concat
    concat: {
      options: {
        separator: ';'
      },

      dist: {}
    },

    // Uglify
    uglify: {
      dist: {}
    },

    //Cssmin
    cssmin: {
      dist: {}
    },

Answer 1

I have solved it. The trouble was caused by:

          options: {
            separator: ';'
          },

in concat.