title: SQL Server 2016 - Error 15150 trying change usermapping
body: <p>I'm trying to add db_backupuperator to a UserMapping but I can't. It returns the Error 15150 and the message:</p>
  
  <p>"Drop failed for User 'dbo' (Microsoft.SqlServer.Smo)
  Additional information: 
  an exception occurred while executing a Transact-SQL statment or batch
  (Microsoft.SqlServer.ConnectionInfo)"</p>
  
  <p>I've already tried apply these sugested solutions:
  <a href="https://blog.sqlauthority.com/2017/12/12/sql-server-fix-msg-15150-cannot-alter-user-dbo/" rel="nofollow noreferrer">https://blog.sqlauthority.com/2017/12/12/sql-server-fix-msg-15150-cannot-alter-user-dbo/</a></p>
  
  <pre><code>Use database_name
  sp_changedbowner 'sa'
  </code></pre>
  
  <p>I've tried also delete the database and create a new one, but then I can't edit any option of User Mapping, can't even add db_datareader or db_datawriter</p>