SQL Server 2005 "Subsystem Invalid" Error Message

If you get the following error message when trying to add a maintenance plan to Microsoft SQL Server 2005:

The specified '@subsystem' is invalid (valid values are returned by sp_enum_sqlagent_subsystems).

Then most likely you're missing SSIS (SQL Server Integration Services) on the server. Install/Reinstall SSIS and see if the problem persists.