diff --git a/winum.el b/winum.el
index 5ef6070..6088f4f 100644
--- a/winum.el
+++ b/winum.el
@@ -258,7 +258,9 @@ Needed to detect scope changes at runtime.")
   "Jump to window 0, or window 10 if 0 is not assigned.
 If prefix ARG is given, delete the window instead of selecting it."
   (interactive "P")
-  (let ((n (if (winum-get-window-by-index 0) 0 10)))
+  (let ((n (or (and (winum-get-window-by-index 0) 0)
+               (and (winum-get-window-by-index 10) 10)
+               (user-error "No window with index 0 or 10"))))
     (if arg
         (winum-delete-window-by-index n)
       (winum-select-window-by-index n))))