That statement works ok for me.
Take a look at your Package.swift file and see what version of fluent-postgresql.git you are using. It looks like that version of the decode function was added June 7, 2018 so you might need to update your Package.swift file and/or run “vapor update” to fetch the updated dependencies.
@axlroach just to confirm what @rcasey said it should definitely work with the Fluent release, make sure you dependencies are up to date and it should be good!