Very Strange Data Conversion (?) Error

 Using SQL Server 2005, I hve encountered a very strange error which I cannot figure out.  The simplified stored proc below basically does a select and sorts the records based on the column name given in the @SortCol parameter.  When I enter the field names bapk, jhpk, etc everything works fine.  The records are returned sorted by the field name entered into the parameter.  The first eight variables are  field names and types

 When I enter the field name 'descr' into the @SortCol variable/parameter, I receive the error:
 Msg 295, Level 16, State 3, Procedure CMSCheck2, Line 28
Conversion failed when converting character string to smalldatetime data type.
I don't see where this is coming from at all.  The problem is in the  stored proc lines beginning with 'CASE @SortCol'
If I take out the lines and just use  OVER (Order By descr) as RowNum   -- everything is fine
If I delete the line  WHEN 'descr' THEN descr  -- everything is fine, it just falls through to the default.

 What is the problem?  Where is a conversion to smalldatetime coming from?   I've been working on this one for a while, and cannot figure it out.  It must be something simple.

 Thanks,
Mike Thomas
 
Below is the stored proc and a testing script
 
TESTING SCRIPTUSE biz03
GO

DECLARE
  @bapk int,
  @jhpk int,
  @transdate smalldatetime,
  @number smallint,
  @descr nvarchar(50),
  @debit decimal(9,2),
  @credit decimal(9,2),
  @cmonth tinyint,
  @StartRowIndex int,
  @MaxRows int,
  @AlphaChar varchar(1),
  @SortCol varchar(20)


SET @MaxRows = 10
SET @StartRowIndex = 0
SET @SortCol = 'descr'
SET @AlphaChar = ''

EXEC [dbo].[CMSCheck2]
  @bapk,
  @jhpk,
  @transdate,
  @number,
  @descr,
  @debit,
  @credit,
  @cmonth,
  @StartRowIndex,
  @MaxRows,
  @AlphaChar,
  @SortCol


 
STORED PROC 
 
 ALTER PROCEDURE [dbo].[CMSCheck2]
  @bapk int,
  @jhpk int,
  @transdate smalldatetime,
  @number smallint,
  @descr nvarchar(50),
  @debit decimal(9,2),
  @credit decimal(9,2),
  @cmonth tinyint,
  @StartRowIndex int,
  @MaxRows int,
  @AlphaChar varchar(1) = null,
  @SortCol varchar(20) = null

AS
BEGIN

SET NOCOUNT ON

DECLARE @lPaging bit
IF @AlphaChar is null
  SET @lPaging = 0
ELSE
  SET @lPaging = 1;

WITH BankListTemp AS
  (SELECT   bapk, jhpk, transdate, number,
    descr, debit, credit, cmonth, ROW_NUMBER()
    OVER (ORDER BY
    CASE @SortCol
       WHEN 'bapk' THEN bapk
       WHEN 'jhpk' THEN jhpk
       WHEN 'transdate' THEN transdate
       WHEN 'number' THEN number
       WHEN 'descr' THEN descr  -- problem here   --
       WHEN 'debit' THEN debit
       WHEN 'credit' THEN credit
       WHEN 'cmonth' THEN cmonth
       ELSE bapk
    END) as RowNum
  FROM bank)

SELECT TOP (@MaxRows)    bapk, jhpk, transdate, number,
    descr, debit, credit, cmonth, Rownum
FROM
 ( SELECT BankListTemp.*,
    (SELECT COUNT(*) FROM BankListTemp) AS RecCount
      FROM BankListtemp) Bank


END