diff --git a/lisp/cus-face.el b/lisp/cus-face.el
index 5dcb2842a2..21fe89c621 100644
--- a/lisp/cus-face.el
+++ b/lisp/cus-face.el
@@ -175,6 +175,7 @@ custom-face-attributes
 		   (choice :tag "Style"
 			   (const :tag "Raised" released-button)
 			   (const :tag "Sunken" pressed-button)
+			   (const :tag "Flat"   flat-button)
 			   (const :tag "None" nil))))
      ;; filter to make value suitable for customize
      (lambda (real-value)