diff -r d682b4000a77 -r f8e52c0152fe src/Pure/Concurrent/event_timer.ML
--- a/src/Pure/Concurrent/event_timer.ML	Sat Apr 04 21:38:20 2020 +0200
+++ b/src/Pure/Concurrent/event_timer.ML	Sun Apr 05 13:05:40 2020 +0200
@@ -130,7 +130,7 @@
 fun manager_check manager =
   if is_some manager andalso Thread.isActive (the manager) then manager
   else
-    SOME (Standard_Thread.fork {name = "event_timer", stack_limit = NONE, interrupts = false}
+    SOME (Isabelle_Thread.fork {name = "event_timer", stack_limit = NONE, interrupts = false}
       manager_loop);
 
 fun shutdown () =