diff --git a/lisp/progmodes/which-func.el b/lisp/progmodes/which-func.el
index e928999ab5e..870db91166a 100644
--- a/lisp/progmodes/which-func.el
+++ b/lisp/progmodes/which-func.el
@@ -110,7 +110,8 @@ activation of Which Function until Imenu is used for the first time."
 (defcustom which-func-update-delay
   ;; Backwards-compatibility: if users had changed this before
   ;; `idle-update-delay' was declared obsolete, let's respect that.
-  (if (/= idle-update-delay 0.5) idle-update-delay 0.5)
+  (with-suppressed-warnings ((obsolete idle-update-delay))
+    (if (/= idle-update-delay 0.5) idle-update-delay 0.5))
   "Idle time delay before `which-function-mode` updates its display.
 When point moves, wait this many seconds after Emacs becomes idle before
 doing an update."