diff --git a/README.org b/README.org
index a89e2b9..3d9d3d2 100644
--- a/README.org
+++ b/README.org
@@ -538,6 +538,14 @@ previous input in the minibuffer.
 
   (add-hook 'minibuffer-setup-hook 'minibuffer-setup-history-completions)
 
+  ;; Stolen from Emacs-28.1 for Emacs-27.2:
+  (unless (fboundp 'minibuffer--completion-prompt-end)
+    (defun minibuffer--completion-prompt-end ()
+      (let ((end (minibuffer-prompt-end)))
+        (if (< (point) end)
+            (user-error "Can't complete in prompt")
+          end))))
+
   ;; Adapted from ‘minibuffer-complete’:
   (defun my-minibuffer-complete-history ()
     (interactive)