Seaquel
SQL Server error code

SQL Server error 6991

SQL Server raises error 6991 at severity 16, which marks a problem with the query or data that you can correct.

Invalid restriction for type '%s'. If the base type has empty content, then the derived type must as well, and if the derived type has empty content, then the base type must be emptiable.
Msg6991
Severity16
Group6000–6999

Source: Database Engine events and errors