Trying To Enforce Ref Integrity On Hierarchy Table Relationship

I've got a table that includes:CREATE TABLE [dbo].[Content] (
  [Id] int IDENTITY(1, 1) NOT NULL,
  [ParentId] int NULL,

I'm wanting to make sure that a ParentId must be in the table as Id someplace else.  When I try to do it by making it a foreign key  get the error:

--------------- SQL ---------------

ALTER TABLE [dbo].[Content]
ADD CONSTRAINT [Content_fk3] FOREIGN KEY ([Id])
  REFERENCES [dbo].[Content] ([ParentId])
  ON UPDATE NO ACTION
  ON DELETE NO ACTION
GO


---------- ERROR MESSAGE ----------

There are no primary or candidate keys in the referenced table 'dbo.Content' that match the referencing column list in the foreign key 'Content_fk3'.
Could not create constraint. See previous errors.

 

Any ideas? 

 

ALTER TABLE [dbo].[Content]
ADD CONSTRAINT [Content_fk3] FOREIGN KEY ([Id])
  REFERENCES [Content].[dbo] ([ParentId])
  ON UPDATE NO ACTION
  ON DELETE NO ACTION
GO