Cannot add diagram to SQL Server 2005 DB: Database diagram support objects cannot be installed because this database does not have a valid owner.

I tried the above mentioned solution but still was not working and displaying the same error message - if you got the same error as me then just try the follwing lines in SQL Query:

EXEC sp_dbcmptlevel 'dbname', '90';

ALTER AUTHORIZATION ON DATABASE::dbname TO valid_login