Laravel 5.1 eloquent Unsupported operand types

I am trying to get query result to array and show them in select element. When I hardcode it like this, it works fine:

$categories = ['Editorial', 'Product test', 'News', 'Feature'];
$categories = ['select_category']+$categories;  

return view('admin.articles.create', compact('categories'));

But when I try to get categories form DB, I get the above mentioned error.

$categories = Category::all();
$categories = ['select_category']+$categories;

return view('admin.articles.create', compact('categories'));

via Chebli Mohamed