Accessing an ArrayAccess item directly

Question

I'm trying to get an item of an array by its key. The array is in fact a SolrObject, which implements ArrayAccess, so I understand that I can access its items as I would in an array.

This is the structure of my $myobject:

SolrObject Object
(
    [116809] => SolrObject Object
    (
        [title] => Array
        (
            [0] => First value
        )

        [description] => Array
        (
            [0] => Second value
        )

    )

    [591978] => SolrObject Object
    (
        [title] => Array
        (
            [0] => Third value
        )

        [description] => Array
        (
            [0] => Forth value
        )
    )
)

So, if I try

var_dump($myobject[116809]);

I get NULL.

But if I try

echo array_key_exists(116809, $myobject);

I get 1

If I try to iterate $myobject with a foreach, I can access its elements correctly.

Why can't I access directly one element by its key?

EDIT: Answering to @ircmaxell, this is the var_dump:

object(SolrObject)#219 (50) {
  [116809]=>
  object(SolrObject)#220 (2) {
    ["title"]=>
    array(1) {
      [0]=>
      string(43) "First value"
    }
    ["description"]=>
    array(1) {
      [0]=>
      string(82) "Second value"
    }
  }
  [591978]=>
  object(SolrObject)#221 (2) {
    ["title"]=>
    array(1) {
      [0]=>
      string(104) "Third value"
    }
    ["description"]=>
    array(1) {
      [0]=>
      string(95) "Fourth value"
    }
  }
}

And this is the result of var_dump($myobject->getPropertyNames());

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