Add member failed for DatabaseRole 'db_datareader'

I tried to give myself datareader and datawriter for my domain login and got this error below. I'm logged into the server with my domain account which has admin rights.

TITLE: Microsoft SQL Server Management Studio

Add member failed for DatabaseRole 'db_datareader'. (Microsoft.SqlServer.Smo)

For help, click: http://go.microsoft.com/fwlink?ProdName=Microsoft+SQL+Server&ProdVer=9.00.1399.00&EvtSrc=Microsoft.SqlServer.Management.Smo.ExceptionTemplates.FailedOperationExceptionText&EvtID=Add+member+DatabaseRole&LinkId=20476

ADDITIONAL INFORMATION:

An exception occurred while executing a Transact-SQL statement or batch. (Microsoft.SqlServer.ConnectionInfo)

Cannot use the special principal 'dbo'. (Microsoft SQL Server, Error: 15405)

For help, click: http://go.microsoft.com/fwlink?ProdName=Microsoft+SQL+Server&ProdVer=09.00.1399&EvtSrc=MSSQLServer&EvtID=15405&LinkId=20476

BUTTONS:

OK

Because you are conected as a sysadmin, you are also the dbo of the database. You cannot add dbo as a member of any fixed role. dbo already has the permissions of db_datareader and db_datawriter.

Thanks
Laurentiu

|||Thanks!