I have issue with commerce_order_load_multiple function

The way i am using this function is

foreach ($mms as $mm) {
$orders_of_user += commerce_order_load_multiple(array(), array('uid' => $mm->uid, 'status' => 'cart'));
}

But i am not getting anything output from this and when i use static value of integer in place of uid i am getting result..

 $orders_of_user += commerce_order_load_multiple(array(), array('uid' => 421, 'status' => 'cart'));

I don’t understand what causing the problem in foreach

foreach ($mms as $mm) {
  dpm($mm);
}