Its wrong design but Why not?

SET ANSI_NULLS ON

GO

SET QUOTED_IDENTIFIER ON

GO

CREATE TABLE [dbo].[Table1](

[PKID] [int] NOT NULL,

[FKID] [int] NULL,

CONSTRAINT [PK_Table1] PRIMARY KEY CLUSTERED

(

[PKID] ASC

)WITH (PAD_INDEX = OFF, IGNORE_DUP_KEY = OFF) ON [PRIMARY]

) ON [PRIMARY]

GO

ALTER TABLE [dbo].[Table1] WITH CHECK ADD CONSTRAINT [FK_Table1_Table2] FOREIGN KEY([FKID])

REFERENCES [dbo].[Table2] ([PKID])

GO

ALTER TABLE [dbo].[Table1] CHECK CONSTRAINT [FK_Table1_Table2]

SET ANSI_NULLS ON

GO

SET QUOTED_IDENTIFIER ON

GO

CREATE TABLE [dbo].[Table2](

[PKID] [int] NOT NULL,

[FKID] [int] NULL,

CONSTRAINT [PK_Table2] PRIMARY KEY CLUSTERED

(

[PKID] ASC

)WITH (PAD_INDEX = OFF, IGNORE_DUP_KEY = OFF) ON [PRIMARY]

) ON [PRIMARY]

GO

ALTER TABLE [dbo].[Table2] WITH CHECK ADD CONSTRAINT [FK_Table2_Table1] FOREIGN KEY([FKID])

REFERENCES [dbo].[Table1] ([PKID])

GO

ALTER TABLE [dbo].[Table2] CHECK CONSTRAINT [FK_Table2_Table1]

GO

Insert into Table2

Select 1,NULL

The commands execute successfully.

Am I missing something?