src/Pure/Concurrent/future.ML

 
 (* results *)
 
 fun error_msg pos ((serial, msg), exec_id) =
   Position.setmp_thread_data pos (fn () =>
-    if is_none exec_id orelse exec_id = Position.get_id pos
-    then Output.error_msg' (serial, msg) else warning msg) ();
+    let val id = Position.get_id pos in
+      if is_none id orelse is_none exec_id orelse id = exec_id
+      then Output.error_msg' (serial, msg) else ()
+    end) ();
 
 fun identify_result pos res =
   (case res of
     Exn.Exn exn =>
       let val exec_id =