src/Pure/Tools/server_commands.scala

       yield Args(build = build, print_mode = print_mode)
 
     def command(
       args: Args,
       progress: Progress = new Progress,
-      log: Logger = No_Logger
+      log: Logger = new Logger
     ) : (JSON.Object.T, (UUID.T, Headless.Session)) = {
       val (_, _, options, session_background) =
         try { Session_Build.command(args.build, progress = progress) }
         catch { case exn: Server.Error => error(exn.message) }