Must declare the scalar variable

Must declare the scalar variable "@ArticleN".

/****** Object:  StoredProcedure [dbo].[vs_Models_SELECT]    Script Date: 11/20/2012 10:45:08 ******/
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
ALTER PROCEDURE [dbo].[Models]

AS


	SET NOCOUNT OFF;

Declare @CategoryID int
Declare @SubCategoryID int
Declare @SortExpression varchar(100)
Declare @Order varchar(100)
Declare  @ArticleN varchar(150)

set @CategoryID=6  
set @SubCategoryID=1   
set @SortExpression=NULL
set @Order=NULL


if @SortExpression is NULL 
Begin
set @SortExpression='p.SeriesId  '
End

if @Order is NULL 
Begin
set @Order ='Desc  '
End

declare @Sort varchar(200)
set @Sort=@SortExpression + @order
set @ArticleN=N'A P I INDIA';

declare @QueryString nvarchar(max)

		set @QueryString= ' Select  --ProductID ,
				Article_No as Model,
				p.SeriesId,
				s.SeriesDesc,
				p.CategoryId,
				ca.CategoryDesc,
				p.MaterialUsedId,
				m.MaterialUsedDesc,
				p.Size, 
				p.ColorId,
				c.ColorNameDesc,
				p.Gender,
				MRP ,
				p.Manufactured_In,
				Maunfacturing_Date,
				Input_Pairs,
				Soldout_Pairs,
     			Remaining_Pairs

		from tabproduct P
		inner join tblkpSeries s
		on p.SeriesId=s.SeriesId
		inner join tblkpCategory ca
		on p.CategoryId=ca.CategoryId
		inner join tblkpMaterialUsed m
		on p.MaterialUsedId=m.MaterialUsedId

		inner join tblkpColor c
		on p.ColorId=c.ColorId

		where 
   		  
		Manufactured_In=case when  @ArticleN  is null then Manufactured_In
			else @ArticleN
			end
	

		Order by ' + @Sort

PRINT @QueryString
 EXEC (@QueryString)