Add member failed for DatabaseRole 'db_securityadmin' sql-server 2008

TITLE: Microsoft SQL Server Management Studio
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Add member failed for DatabaseRole 'db_securityadmin'.  (Microsoft.SqlServer.Smo)
For help, click: http://go.microsoft.com/fwlink?ProdName=Microsoft+SQL+Server&ProdVer=10.50.1600.1+((KJ_RTM).100402-1539+)&EvtSrc=Microsoft.SqlServer.Management.Smo.ExceptionTemplates.FailedOperationExceptionText&EvtID=Add+member+DatabaseRole&LinkId=20476
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ADDITIONAL INFORMATION:
An exception occurred while executing a Transact-SQL statement or batch. (Microsoft.SqlServer.ConnectionInfo)
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Cannot use the special principal 'dbo'. (Microsoft SQL Server, Error: 15405)
For help, click: http://go.microsoft.com/fwlink?ProdName=Microsoft+SQL+Server&ProdVer=10.50.1600&EvtSrc=MSSQLServer&EvtID=15405&LinkId=20476
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BUTTONS:
OK
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