diff -r 83a2b6976515 -r c3589f2dff31 src/Pure/GUI/gui_thread.scala
--- a/src/Pure/GUI/gui_thread.scala	Sat Jan 09 22:41:08 2021 +0100
+++ b/src/Pure/GUI/gui_thread.scala	Sun Jan 10 13:04:29 2021 +0100
@@ -22,7 +22,7 @@
 
   def require[A](body: => A): A =
   {
-    Predef.require(SwingUtilities.isEventDispatchThread)
+    Predef.require(SwingUtilities.isEventDispatchThread, "GUI thread expected")
     body
   }