How to fix SQL Server database diagram error "Cannot execute as the database principal because the principal 'dbo' does not exist"?

When attempting to create a database diagram I get the following error:

Cannot execute as the database principal because the principal "dbo" does not exist

How do I fix this error?

This error occurs when the database owner for the files setting in the database properties is unassigned (or blank).

To fix this:

• Right click on the database you need to create a diagram for and choose Properties.
• Choose Files from the left hand navigation menu of the Database Properties dialog box.
• Enter sa in the owner field.
• Click OK

This gives the sa permission to create files on the server.

You can also run this query to do the same thing:

EXEC sp_changedbowner 'sa';

src: SQL SERVER – Fix Error – Cannot execute as the database principal because the principal “dbo” does not exist