diff --git a/src/xfns.c b/src/xfns.c
index 70a4b6d5509..f960f36e24d 100644
--- a/src/xfns.c
+++ b/src/xfns.c
@@ -6648,7 +6648,8 @@ Internal use only, use `display-monitor-attributes-list' instead.  */)
 #else
 	  i = gdk_screen_get_monitor_at_window (gscreen, gwin);
 #endif
-	  ASET (monitor_frames, i, Fcons (frame, AREF (monitor_frames, i)));
+	  if (0 <= i && i < n_monitors)
+	    ASET (monitor_frames, i, Fcons (frame, AREF (monitor_frames, i)));
 	}
     }