访问阵列无法正常工作

Okay, this seems weird to me!

I have an array with user objects saved in it using the user id as the array key

Array (
    [75] => pwUser Object
        (
            [data:protected] => Array
                (
                    [ID] => 75
                    [display_name] => Chris Waa
                )
        )

    [1] => pwUser Object
        (
            [data:protected] => Array
                (
                    [ID] => 1
                    [display_name] => Alex Müller
                )
        )

)

I now want to access this array with the following code

$this->users[$notification['subscribeNotID']]->getAvatar()

But I'm receiving an error

Notice: Undefined index: in NotificationList.class.php on line 217

Fatal error: Call to a member function getAvatar() on a non-object in NotificationList.class.php on line 217

Okay, let's var_dump $notification['subscribeNotID']

string(2) "75"

The key 75 is definitely available in the array! Curiously, the script works like that

$ID = "75"; // var_dump() returns the same as $notification["subscribeNotID"] above
$this->users[$ID]->getAvatar()

Why?