diff --git a/lisp/register.el b/lisp/register.el
index 31214ae3ba0..b0ee759ec5b 100644
--- a/lisp/register.el
+++ b/lisp/register.el
@@ -146,7 +146,10 @@ all; the preview buffer is still accessible with `help-char' (\\`C-h').
 
 When set to `traditional' (the default), provide a more basic preview
 according to `register-preview-delay'; this preserves the traditional
-behavior of Emacs 29 and before."
+behavior of Emacs 29 and before.
+
+Setting this variable with `setq' has no effect; use either `setopt'
+or `customize-option' to change its value."
   :type '(choice
           (const :tag "Use preview" t)
           (const :tag "Use preview and exit by pressing register name" insist)