src/Pure/Tools/build.ML

   let
     val _ = SHA1.test_samples ();
     val _ = Options.load_default ();
     val _ = Isabelle_Process.init_options ();
     val args = decode_args (File.read (Path.explode args_file));
-    fun error_message msg = writeln ("\ferror_message = " ^ encode_lines msg);
+    fun error_message msg = writeln ("\ferror_message = " ^ encode_lines (YXML.content_of msg));
     val _ =
       Unsynchronized.setmp Private_Output.protocol_message_fn protocol_message
         build_session args
       handle exn => (List.app error_message (Runtime.exn_message_list exn); Exn.reraise exn);
     val _ = Options.reset_default ();