quantity_discount module

Hi, I am currently working on a ecommerce website and I was looking for a module allowing me to assign a price on a product according to a specified amount (2 € from 1 to 99, 1.9 € to 100 to 500 etc. ...).

The worry is that after install the quantity_discount module, but after that, the "?q=cart/view" page tell me : "warning: Invalid argument supplied for foreach () in [...] \ modules \ quantity_discount \ quantity_discount.module on line 418. "

After research it seems that on this famous line: foreach ($form["items"] as &$item)

$form["item"] seems not to be an array ??

thanks,

Martouff

ps: scuze my poor english !! (im french)