SPL ArrayAccess offsetExists default behavior

Hi list, The following doesn't strike me as consistent behavior:

$a           = new ArrayObject();

$a["foobar"] = NULL; echo (int) isset( $a["foobar"] ); // Output: 1 While it's technically correct, I find it confusing. The reason is probably because of a array_key_exists (or property_exists) check within offsetExists() that do not take return false on a NULL value, unlike isset(). What say you ? - Mark