Lookup Transform Still Cannot Find Empty String Matches

Has anyone find solution for this problem.
i also checked
http://forums.microsoft.com/MSDN/ShowPost.aspx?PostID=298056&SiteID=1 and
http://blogs.conchango.com/kristianwedberg/archive/2006/02/22/2955.aspx


Suppose have a Dimension table

DimColor
----------------------------
ColorKeyPK(smallint) ColorAlternateKey(nvarchar(30))
-1 UnknownMember
1
2 Blue
3 Red
4 Black

Color with the ID 1 is empty string

FactOrders
---------------------------
OrderID Date Color Quantity

OrderID = 1 Color = 'Black' Quantity = 10
OrderID = 2 Color = 'Red' Quantity = 20
OrderID = 3 Color = '' Quantity = 10
OrderID = 4 Color = 'Blue' Quantity = 5
OrderID = 5 Color = Black Quantity = 10

When i use the Lookup transform it cannot find the ColorKeyPK

The result of the Lookup transform is.
------------------------------
OrderID = 1 Color='Black' ColorKey=4
OrderID = 2 Color='Black' ColorKey=3
OrderID = 3 Color='Black' ColorKey=NULL ----> This is the problem Lookup cannot find empty string. It should be 1.
OrderID = 4 Color='Black' ColorKey=2
OrderID = 5 Color='Black' ColorKey=4

Thanks from now.