--- libev/ev.c	2019/06/25 04:52:30	1.497
+++ libev/ev.c	2019/06/26 00:01:46	1.498
@@ -3925,6 +3925,9 @@
   assert (("libev: ev_io_start called with negative fd", fd >= 0));
   assert (("libev: ev_io_start called with illegal event mask", !(w->events & ~(EV__IOFDSET | EV_READ | EV_WRITE))));
 
+#if EV_VERIFY >= 2
+  assert (("libev: ev_io_start called on watcher with invalid fd", fd_valid (fd)));
+#endif
   EV_FREQUENT_CHECK;
 
   ev_start (EV_A_ (W)w, 1);
@@ -3950,6 +3953,9 @@
 
   assert (("libev: ev_io_stop called with illegal fd (must stay constant after start!)", w->fd >= 0 && w->fd < anfdmax));
 
+#if EV_VERIFY >= 2
+  assert (("libev: ev_io_stop called on watcher with invalid fd", fd_valid (w->fd)));
+#endif
   EV_FREQUENT_CHECK;
 
   wlist_del (&anfds[w->fd].head, (WL)w);