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;
        }
}