CI 3.0.1 bug with form submit

When I using form submit it has returned weird result.



a input key name is 'UTF-8' char set string



and here is my code in view file



<form action="http://localhost/array_test/post_print" method="post">

    <input type="hidden" name="アオ" value="blue">
    <input type="hidden" name="アカ" value="red">
    <input type="submit">
</form>



and this is a 'post_print' function in array_test file



function post_print()
{
   print_r($_POST);
}



JUST ONLY PRINT_R!



and it has return 



Array
(
   [アオ] => blue
   [アカ] => red
   [0] => red
)



What is [0] key? Where did that came from?



It has appeared with last UTF-8 key's value when I using UTF-8 key.



Is this a bug?