UNPIVOT with NULL values

Question

I have the following query : 

SELECT Custid, Shipperid, Freight
FROM Sales.FreightTotals
UNPIVOT( Freight FOR Shipperid IN([1],[2],[3]) ) AS U;

I have the following result :

 

custid    shipperid    freight

1              1              98

1              3              55

2              2              75

 

This is my expected output :

 

custid    shipperid    freight

1              1              98

1              2              NULL

1              3              55

2              1              NULL

2              2              75

2              3              NULL