Optional Input Parameter in Stored Proc kills performance

Hey All -

I have the stored procedure whose performance dies when I use an input parmeter set to NULL:

ALTER PROCEDURE [dbo].[TEST]
   (
	@AcctgPeriod int = NULL,
         @ReturnValue As int OUTPUT,
	@RecordCount As int OUTPUT,
	@ErrMsg AS nVarChar(MAX) OUTPUT
	)
AS
BEGIN

When "@AcctgPeriod int = NULL" is changed to "@AcctgPeriod int = -1" then the thing runs great.

Can't find anything to explain this.

Other informaiton ... using SQL Server 2008 r2.