How to create a source map for my compiled JS files with mincer?

Question

I want to compile some coffeescript files into one JS file using mincer and Gulp. I have an app.coffee file which requires all other files. My gulp task uses gulp-mincer and looks like that:

var gulp = require('gulp');
var Mincer = require('mincer');
var mince = require('gulp-mincer');

gulp.task('mincer', function(){
  var env = new Mincer.Environment();
  env.enable('source_maps');
  env.appendPath('src/coffee');

  return gulp.src('src/coffee/**/app.*')
    .pipe(mince(env))
    .pipe(gulp.dest('public/js'));
});

If I run this task I get one JS file with all required code in it but no source map. What I have to do to get also the source map file?