If i run this code, everything works fine:

gulp.task('zip-build', function() {
  return gulp.src([
    '../admin/web/css/diamond-matrix-widget.css',
    '../admin/web/js/diamond-matrix-widget.js'
  ])
  .pipe(zip('diamond-matrix-widget.zip'))
  .pipe(gulp.dest('../admin/web'))
});

However if i include the .js file first and .css file after in the src array then the .css file is empty when zipped. Quite strange.

Just thought i'd mention in case its a bug?

Running on:
OSX 10.11.6
gulp 3.9.1
gulp-zip 4.0.0