src/Pure/Concurrent/isabelle_thread.scala

--- a/src/Pure/Concurrent/isabelle_thread.scala	Sat Jan 09 22:41:08 2021 +0100
+++ b/src/Pure/Concurrent/isabelle_thread.scala	Sun Jan 10 13:04:29 2021 +0100
@@ -179,7 +179,7 @@
   def interrupt_handler[A](new_handler: Isabelle_Thread.Interrupt_Handler)(body: => A): A =
     if (new_handler == null) body
     else {
-      require(is_self)
+      require(is_self, "interrupt handler on other thread")
 
       val old_handler = handler
       handler = new_handler