diff -r d1538d4de057 -r f871ccd358b3 src/Pure/Concurrent/isabelle_thread.scala
--- a/src/Pure/Concurrent/isabelle_thread.scala	Mon Apr 06 21:22:42 2020 +0200
+++ b/src/Pure/Concurrent/isabelle_thread.scala	Mon Apr 06 21:58:24 2020 +0200
@@ -17,7 +17,7 @@
   def self: Isabelle_Thread =
     Thread.currentThread match {
       case thread: Isabelle_Thread => thread
-      case _ => error("Isabelle-specific thread required")
+      case thread => error("Isabelle-specific thread required: " + thread)
     }
 
   def check_self: Boolean =