I have some notice in a line 27 of the method bellow, and can't find the issue !

    public function blockOrder()
    {
        $items = Mage::getSingleton('checkout/cart')->getQuote()->getItemsCollection();
        $tab = array();
        foreach ($items as $itemCart) {
        /*line 27*/ $tab[$itemCart->getUdropshipVendor()] += $itemCart->getPrice() * $itemCart->getQty();
        }
        foreach ($tab as $vendorId => $amountInCart) {
            $vendorMinAmount = Mage::getModel('udropship/vendor')->load($vendorId)->getMinimumOrderAmount();
            if ($amountInCart < $vendorMinAmount) {
                return true;
            }
            return false;
        }
    }