Sourcemaps not working

[thany]

Sourcemaps aren't being outputted when I use amd-optimize. When I leave it out, sourcemaps work fine.

return gulp.src(['source/js/**/*.js'])
    .pipe(sourcemaps.init())
      .pipe(amdOptimize('js/main', config))  //<-- comment to get sourcemaps to work
      .pipe(concat('main.js'))
    .pipe(sourcemaps.write('./'))
    .pipe(gulp.dest('source/js/main'));