Codeigniter POST数据名称

I have a problem with post data variable names for example i might get this type of input:

<input type="number" name="amount[BKP1P+NC10A]" />

Which in theory there is nothing wrong with the array element name and when i am not in codeigniter i receive the array name and access it as usual but strange is when i am using the $this->input->post() or even when i access the data directly from $_POST the array element name is removed and instead i get an enumerative array.

For example in normal post (without codeigniter) i get:

 array(1) {
  ["amount"]=>
   array(1) {
     ["BKP1P+NC10A"]=>
     string(2) "12"
   }
 }

While in codeigniter i get this:

array(1) {
  ["amount"]=>
   array(1) {
    [0]=>
     string(2) "12"
  }
}

Can anyone tell me whats going on and how i can get the actual name instead of an enumerative array?

Thanks & Regards

It seems that the problem as @Goose pointed out is from the core of codeigniter and as i revisited the core input class i found a function _clean_input_keys which contains:

preg_match('/^[a-z0-9:_\/|-]+$/i', $str);

As you can see + and . are not allowed so i changed it to:

preg_match('/^[a-z0-9\+\.:_\/|-]+$/i', $str);

Hope this helps anyone!