diff -r e5e34bd28257 -r f9f7c34b7dd4 src/Pure/System/invoke_scala.ML
--- a/src/Pure/System/invoke_scala.ML	Sat Nov 02 10:56:53 2019 +0100
+++ b/src/Pure/System/invoke_scala.ML	Sat Nov 02 12:02:27 2019 +0100
@@ -33,7 +33,7 @@
     fun abort () = Output.protocol_message (Markup.cancel_scala id) [];
     val promise = Future.promise_name "invoke_scala" abort : string future;
     val _ = Synchronized.change promises (Symtab.update (id, promise));
-    val _ = Output.protocol_message (Markup.invoke_scala name id) [arg];
+    val _ = Output.protocol_message (Markup.invoke_scala name id) [XML.Text arg];
   in promise end;
 
 fun method name arg = Future.join (promise_method name arg);