Fail to add user 'sa' as a owner of a database for administration

Dear all,

I try to set permission of "sa" user to a database I created with an other Windows admin account. When trying to set mapping role to owner in order that "sa" account gets all permission on that database I get a failure creation and exception is as follow :

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Create failed for User 'sa'.  (Microsoft.SqlServer.Smo)

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For help, click: http://go.microsoft.com/fwlink?ProdName=Microsoft+SQL+Server&ProdVer=10.50.1750.9+((dac_inplace_upgrade).101209-1051+)&EvtSrc=Microsoft.SqlServer.Management.Smo.ExceptionTemplates.FailedOperationExceptionText&EvtID=Create+User&LinkId=20476

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Program Location:

   at Microsoft.SqlServer.Management.Smo.SqlSmoObject.CreateImpl()
   at Microsoft.SqlServer.Management.Smo.User.Create()
   at Microsoft.SqlServer.Management.SqlManagerUI.CreateLoginData.LoginPrototype.ApplyDatabaseRoleChanges(Server server)
   at Microsoft.SqlServer.Management.SqlManagerUI.CreateLoginDatabaseAccess.OnRunNow(Object sender)

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

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Program Location:

   at Microsoft.SqlServer.Management.Common.ServerConnection.ExecuteNonQuery(String sqlCommand, ExecutionTypes executionType)
   at Microsoft.SqlServer.Management.Common.ServerConnection.ExecuteNonQuery(StringCollection sqlCommands, ExecutionTypes executionType)
   at Microsoft.SqlServer.Management.Smo.ExecutionManager.ExecuteNonQuery(StringCollection queries)
   at Microsoft.SqlServer.Management.Smo.SqlSmoObject.ExecuteNonQuery(StringCollection queries, Boolean includeDbContext)
   at Microsoft.SqlServer.Management.Smo.SqlSmoObject.CreateImplFinish(StringCollection createQuery, ScriptingOptions so)
   at Microsoft.SqlServer.Management.Smo.SqlSmoObject.CreateImpl()

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Cannot use the special principal 'sa'. (.Net SqlClient Data Provider)

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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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Server Name: .\SQLEXPRESS
Error Number: 15405
Severity: 16
State: 1
Line Number: 1


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Program Location:

   at Microsoft.SqlServer.Management.Common.ConnectionManager.ExecuteTSql(ExecuteTSqlAction action, Object execObject, DataSet fillDataSet, Boolean catchException)
   at Microsoft.SqlServer.Management.Common.ServerConnection.ExecuteNonQuery(String sqlCommand, ExecutionTypes executionType)

Any idea how should I do ?

regards

serge

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