Gulp useref generates empty file

Question

I have a gulp setup with useref to concatenate and minify my css files:

var gulpIf = require('gulp-if');
var useref = require('gulp-useref');
var cssnano = require('gulp-cssnano');

gulp.task('useref', function() {
  return gulp.src('app/*.html')
    .pipe(useref())
    .pipe(gulpIf('*.css', cssnano()))
    .pipe(gulp.dest('dist'))
});

My HTML file looks like:

<!-- build:css assets/css/theme.min.css -->
<link rel="stylesheet" href="assets/css/theme.css">
<!-- endbuild -->

package.json:

"gulp-useref": "^3.1.2",

When I do a new build, the theme.min.css gets created, however it is empty (while the original theme.css has something like 150Kb of styles). No error is thrown too. Any ideas?