diff --git a/src/atimer.c b/src/atimer.c
index c205f658d74..c87b93697b5 100644
--- a/src/atimer.c
+++ b/src/atimer.c
@@ -205,6 +205,9 @@ cancel_atimer (struct atimer *timer)
 	}
     }
 
+  /* We shouldn't be called with timers which aren't on either list.  */
+  eassert (i != 2);
+
   unblock_atimers (&oldset);
 }