How To Write This Query?

hi guys,

I run into this question again. Here is it.
I have two tables like this:
StoreVisit(StoreVisitID(PK), UserID, StoreID, LoginID, LogoutID)
ProductOrders(ProductID, UserID, StoreID, OrderQuantity, StoreVisitID)
What I want to do is, I need to find out the lastest store visit record and put the StoreVisitID into ProductOrders table for the same userID, StoreID. So what I tried is:

update ProductOrders set StoreVisitID=sv.StoreVisitID
from ProductOrders po
inner join (select top 1 * from StoreVisit where UserID=po.UserID and StoreID=po.StoreID order by StoreVisitID desc) sv
on po.StoreID=sv.StoreID and po.UserID=sv.UserID
Where po.UserID=@UID and po.StoreVisitID is null

When I run it, it shows the error:
The column prefix po, does not match with a table name or alias name used in the query.
Looks like it does not recognize the po in select top 1... .

I hope you guys already understand what I am trying to do. I am wondering what is the best way to do this?
Thanks.