OptimisticConcurrencyException with Rows Affected Parameter on an Update Stored Procedure

Question

I have an update sproc (notice I have inserted a RowVersion column of type timestamp in Product from Northwind):

CREATE PROCEDURE [dbo].[ProductUpdate]
    (
    @productId int,
    @productName nvarchar(40),
    @categoryID int,
    @unitPrice money,
    @discontinued bit,
    @rowversion timestamp,
    @updateCount int OUTPUT
    )
AS
    UPDATE Products
    SET ProductName = @productName,
        CategoryID = @categoryID,
        UnitPrice = @unitPrice,
        Discontinued = @discontinued
    WHERE ProductID = @productId
    AND RowVersion = @rowversion

    SELECT RowVersion
    FROM Products
    WHERE ProductID = @productId
    AND @@ROWCOUNT > 0

    SELECT updateCount = @@ROWCOUNT

    RETURN

I mapped the update sproc to the Product entity, checking "Use Original Value" for RowVersion and "Rows Affected Parameter" for @updateCount.

For some reason I am getting an OptimisticConcurrencyException when I should not.  Am I doing something wrong?  My sample project is here:
http://tonysneed.com/elinq/download/HelloSprocs.zip

Cheers,
Tony