diff --git a/winum.el b/winum.el
index 83f156a..8e35505 100644
--- a/winum.el
+++ b/winum.el
@@ -88,7 +88,8 @@ numbers in the mode-line.")
 
 (defcustom winum-ignored-buffers '(" *which-key*")
   "List of buffers to ignore when selecting window."
-  :type '(repeat string))
+  :group 'winum
+  :type  '(repeat string))
 
 (defface winum-face '()
   "Face used for the number in the mode-line."