SSP: Adding Items Error

I am creating a .ss function that will add an item to an order if it isn't currently in the order.  When the script gets ran, I get the following error message.

"Failed when adding item to cart"

The item exists and can be added from the website fine.  I also used the singular version of addItems(order.addItem(item)), but received the same error.  Any help would be appreciated.

function service(request,response)  {      var shoppingCartItemId = "22";      var order = nlapiGetWebContainer().getShoppingSession().getOrder();      var items = order.getFieldValues({          'items': ['internalid']      });      var hasShoppingCartItem = false;            if (items.items != null) {          for (i = 0; i < items.items.length; i++) {              if (items.items[i].internalid == shoppingCartItemId) {                  hasShoppingCartItem = true;              }          }      }            var itemArray = new Array();            if (!hasShoppingCartItem) {          var item = {              internalid : shoppingCartItemId,              quantity : 1          };          itemArray.push(item);          order.addItems(itemArray);      }            var itemsJSON = JSON.stringify(order.getFieldValues({          'items': ['name', 'salesdesc', 'quantity', 'rate', 'amount', 'options'],           'summary': ['total']      }));            try {          response.setContentType('JSON');          response.writeLine(itemsJSON);      } catch (exception) {           var e2 = nlapiCreateError(e);      }  }