Error Placing Manual Order (Add New)

[contempoweb]

We have a problem when adding a new order manually from the BACK-END of our PrestaShop 1.5.6.2

(Placing order on the front end works fine)

After filling out all details regarding customer details, adding products, and choosing carrier and then clicking  "create order" we get the following error on a blank page:

Fatal error: Call to a member function getOrderTotal() on a non-object in public_html/us/override/classes/Carrier.php on line 78

Line 78 in Carrier.php is this:
$orderTotal = $this->context->cart->getOrderTotal(true, Cart::ONLY_PRODUCTS_WITHOUT_SHIPPING);
 

 

Any idea why this happens?

Edited June 5, 2014 by contempoweb (see edit history)