diff -r c6e9c7d140ff -r c88d1c36c9c3 src/Pure/Concurrent/event_timer.ML
--- a/src/Pure/Concurrent/event_timer.ML	Wed Jun 21 22:57:40 2017 +0200
+++ b/src/Pure/Concurrent/event_timer.ML	Thu Jun 22 14:27:13 2017 +0200
@@ -148,7 +148,7 @@
   let
     val req: request Single_Assignment.var = Single_Assignment.var "request";
     fun abort () = ignore (cancel (Single_Assignment.await req));
-    val promise: unit future = Future.promise abort;
+    val promise: unit future = Future.promise_name "event_timer" abort;
     val _ = Single_Assignment.assign req (request time (Future.fulfill promise));
   in promise end);