SQL Server 2008 R2 Database Diagram Error Code
This can happen if your database was created in a previous version of SQL Server. You need to recreate the table [dbo].[sysdiagrams] or if you do not want to lose already existing diagrams try this:

ALTER table [dbo].[sysdiagrams]
DROP COLUMN [diagram_id]

GO

ALTER table [dbo].[sysdiagrams]
ADD [diagram_id] [int] NOT NULL IDENTITY (1, 1)