Insertion Faild In Partitioned View In Sql Server 7.0

I have a problem while I try to insert data into a partioned view I am
getting the following error.

Server: Msg 4436, Level 16, State 12, Line 9
UNION ALL view 'sales_all' is not updatable because a partitioning column
was not found.

Any thoughts

USE pubs

CREATE TABLE sales_monthly
( sales_month int NOT NULL ,
sales_qty int NOT NULL
)
GO
CREATE TABLE sales_jan
( sales_month int NOT NULL,
sales_qty int NOT NULL
)
GO
CREATE TABLE sales_feb
( sales_month int NOT NULL,
sales_qty int NOT NULL
)
GO

ALTER TABLE sales_feb WITH NOCHECK ADD
CONSTRAINT PK_sales_feb PRIMARY KEY CLUSTERED
( sales_month
) ,
CONSTRAINT CK_sales_feb CHECK (sales_month = 2)
GO

ALTER TABLE sales_jan WITH NOCHECK ADD
CONSTRAINT PK_sales_jan PRIMARY KEY CLUSTERED
( sales_month
) ,
CONSTRAINT CK_sales_jan CHECK (sales_month = 1)
GO