I'm having problems with dynamic sorting using ROW Number in SQL Server. I have it working but it's throwing errors on non numeric fields. What do I need to change to get sorts with Alpha Working???

ID  Description
5   Test    
6   Desert  
3   A evil  

Ive got a Sql Prodcedure

CREATE PROCEDURE [CRUDS].[MyTable_Search]
    -- Add the parameters for the stored procedure here
    -- Full Parameter List  
    @ID int = NULL,     
    @Description nvarchar(256) = NULL,  
    @StartIndex int = 0,
    @Count int = null,
    @Order varchar(128) = 'ID asc'
AS
BEGIN
    -- SET NOCOUNT ON added to prevent extra result sets from
    -- interfering with SELECT statements.
    SET NOCOUNT ON;

    -- Insert statements for procedure here
Select * from
    (
        Select ROW_NUMBER() OVER 
            (Order By
                case 
                    when @Order = 'ID asc' then [TableName].ID  
                    when @Order = 'Description asc' then [TableName].Description    
                end asc,
                case
                    when @Order = 'ID desc' then [TableName].ID 
                    when @Order = 'Description desc' then [TableName].Description   
                end desc
            ) as row,
            [TableName].* from [TableName]
        where 
            (@ID IS NULL OR [TableName].ID = @ID)  AND
            (@Description IS NULL OR [TableName].Description = @Description)
) as a
where 
    row > @StartIndex  
    and (@Count is null or row <= @StartIndex + @Count)
 order by
    case 
        when @Order = 'ID asc' then a.ID    
        when @Order = 'Description asc' then a.Description  
    end asc,
    case
        when @Order = 'ID desc' then a.ID   
        when @Order = 'Description desc' then a.Description 
    end desc


END