Description:
------------
After a debug_backtrace() the result array doesn't have 'object' entry if Zend Optimizer (3.0.1 in this case) is enabled. 
Turn off Zend Optimizer and 'object' appear.

(I've asked on Zend Optimizer forum also, I don't know if a php related problem)

Reproduce code:
---------------
$traces = debug_backtrace();
print_r($traces);

Expected result:
----------------
    [1] => Array
        (
            [file] => /var/www/symfony/lib/controller/sfFrontWebController.class.php
            [line] => 48
            [function] => forward
            [class] => sfController
            [type] => ->
            [args] => Array
                (
                    [0] => story
                    [1] => index
                )

        )

Actual result:
--------------
    [1] => Array
        (
            [file] => /var/www/symfony/lib/controller/sfFrontWebController.class.php
            [line] => 48
            [function] => forward
            [class] => sfController
            [type] => ->
            [object] => !!!!!!!!!!!!!!!!!!!
            [args] => Array
                (
                    [0] => story
                    [1] => index
                )

        )