diff --git a/scheduler.scm b/scheduler.scm
index f89805f..4d95ef6 100644
--- a/scheduler.scm
+++ b/scheduler.scm
@@ -266,7 +266,10 @@ EOF
     (lambda (reason state)
       (when (fx= reason 255)           ; C_TIMER_INTERRUPT_NUMBER
        (let ([ct ##sys#current-thread])
-         (##sys#setslot ct 1 (lambda () (oldhook reason state))) 
+         (##sys#setslot ct 1 (lambda ()
+                               (let ((old-blockobj (##sys#slot ct 11)))
+                                 (oldhook reason state)
+                                 (set! (##sys#slot ct 11) old-blockobj)))) 
          (##sys#schedule) ) )          ; expected not to return!
       (oldhook reason state) ) ) )