DateTime对象 - 500内部服务器错误

When I print_r my array, it shows a series of Date Objects like below:

DateTime Object
(
    [date] => 2011-11-08 00:00:00
    [timezone_type] => 3
    [timezone] => America/New_York
)
DateTime Object
(
    [date] => 2011-11-17 00:00:00
    [timezone_type] => 3
    [timezone] => America/New_York
) 

... etc, etc

But when I try to echo it out, I hit a 500 Error. What gives? I don't see any difference between

print_r($row[$x]['OrderDate']);

and

echo '<td>' . $row[$x]['OrderDate'] . '</td>';

echo $row[$x]['OrderDate']->format('m-d-Y');

echo '<td>' . date('Y-m-d', $row[$x]['OrderDate'] . '</td>'