Seaquel
SQL Server error code

SQL Server error 6500

SQL Server raises error 6500 at severity 16, which marks a problem with the query or data that you can correct.

%ls failed because method '%ls' of class '%ls' in assembly '%ls' returns %ls, but CLR Triggers must return void.
Msg6500
Severity16
Group6000–6999

Source: Database Engine events and errors