diff --git a/cart-grain.go b/cart-grain.go
index 3079d48..ec03340 100644
--- a/cart-grain.go
+++ b/cart-grain.go
@@ -289,7 +289,7 @@ func (c *CartGrain) HandleMessage(message *Message, isReplay bool) (*FrameWithPa
 	case ChangeQuantityType:
 		msg, ok := message.Content.(*messages.ChangeQuantity)
 		if !ok {
-			err = fmt.Errorf("expected RemoveItem")
+			err = fmt.Errorf("expected ChangeQuantity")
 		} else {
 			for i, item := range c.Items {
 				if item.Id == int(msg.Id) {
@@ -297,8 +297,9 @@ func (c *CartGrain) HandleMessage(message *Message, isReplay bool) (*FrameWithPa
 						c.TotalPrice -= item.Price * int64(msg.Quantity)
 						c.Items = append(c.Items[:i], c.Items[i+1:]...)
 					} else {
-						item.Quantity -= int(msg.Quantity)
-						c.TotalPrice -= item.Price * int64(msg.Quantity)
+						diff := int(msg.Quantity) - item.Quantity
+						item.Quantity = int(msg.Quantity)
+						c.TotalPrice += item.Price * int64(diff)
 					}
 
 					break