Custom Ordering Question

Hi group, i'm having a problem with custom ordering in a SP. My code is
the following:

CREATE PROCEDURE [dbo].[PersonasSelectAll_P]
@fromRow int,
@toRow int,
@expresionOrdenamiento int = null
AS
SET NOCOUNT ON

DECLARE @TotalFilas int

Select @TotalFilas = count(*) from [Personas]

SELECT * FROM
(
SELECT ROW_NUMBER() OVER (ORDER BY CASE WHEN @expresionOrdenamiento =
1 THEN [Id]
--WHEN @expresionOrdenamiento = 2 THEN [Nombre_RazonSocial]
WHEN @expresionOrdenamiento = 3 THEN [CUIT_CUIL]
ELSE null
END
) AS Row
, * , @TotalFilas as TotalRows
FROM [Personas]
AS NumberedPersons
WHERE
(Row >= isnull(@fromRow, 0)) AND (Row <= isnull(@toRow, row))


The line WHEN @expresionOrdenamiento = 2 THEN [Nombre_RazonSocial] is
erroring out (the sp compiles, it's a runtime error), i'm getting an
'Cannot convert varchar to int'. Maybe it's because Nombre_RazonSocial
is varchar? If i comment that line then everything works fine. Any
Help??

Thanks in advance