leaving behind a null

in my `gulpfile.js`, i have:

```js
gulp.task('index', function () {
  return gulp.src('app/index.html')
    .pipe(htmlbuild({
      js: htmlbuild.preprocess.js(function (block) {
        block
          .pipe(gulpSrc())
          .pipe(concat('libraries.js'))
          .pipe(gulp.dest(('app/scripts/'));

        block.end('scripts/libraries.js');
      })
    }))
    .pipe(gulp.dest(prefix('app/')));
});
```

where `gulpSrc` is taken from the example in the docs.

in my `index.html`, i have:

```html
<!-- htmlbuild:js -->
<script src="bower_components/angular/angular.js"></script>
<script src="bower_components/angular-animate/angular-animate.js"></script>
<script src="bower_components/angular-sanitize/angular-sanitize.js"></script>
<script src="bower_components/angular-ui-router/release/angular-ui-router.js"></script>
<script src="app/scripts/libraries/hmac-sha1.js"></script>
<script src="node_modules/oauth-1.0a/oauth-1.0a.js"></script>
<!-- endbuild -->
```

and when i run the task, i end up with:

```html
null<script src="scripts/libraries.js"></script>
```

----

where's that leading `null` coming from?