Dynamisk ORDER BY i SP virker ikke

Hvorfor virker den ikke?
Jeg får denne fejlmedelse: "if the SELECT statement contains a UNION operator, the sort columns must appear in the select list"

CREATE PROCEDURE test
(@OrderBy    int)
As

SELECT land, landus  FROM land WHERE landid>0
UNION all
SELECT  land, landus  FROM land WHERE landid>0
ORDER BY Case @OrderBy
WHEN 1 Then cast(land AS sql_variant)
WHEN 2 Then cast(landus AS sql_variant)
End
GO


Peter Lang