How to properly order the collection 'asc'

Question

Im trying to order 'asc' the collection below but its not working with the sortBy collection method. Do you know how to properly achieve that?

 $topCategories  =  PostCategory::pluck('category_name', 'category_icon')->toArray();
        $topCategories =  collect($postInformation['categories']['data'])->map(function ($postCategory) {
            return
                $postCategory['name']['en'];
        })->sortBy('category_name')->toArray();

dd($topCategories) shows like:

^ array:12 [▼
  0 => "Testing category"
  1 => "category 2"
  ...
]

But the category 2 should appear first.