1 files changed, 5 insertions, 0 deletions

diff --git a/lisp/calendar/todo-mode.el b/lisp/calendar/todo-mode.el
index dcc960fb9bf..6e975249a3f 100644
--- a/lisp/calendar/todo-mode.el
+++ b/lisp/calendar/todo-mode.el
@@ -743,6 +743,11 @@ corresponding todo file, displaying the corresponding category."
             (setq todo-category-number (todo-category-number cat)))
           ;; If this is a new todo file, add its first category.
           (when (zerop (buffer-size))
+            ;; Don't confuse an erased buffer with a fresh buffer for
+            ;; adding a new todo file -- it might have been erased by
+            ;; mistake or due to a bug (e.g. Bug#20832).
+            (when (buffer-modified-p)
+              (error "Buffer is empty but modified, please report a bug"))
             (let (cat-added)
               (unwind-protect
                   (setq todo-category-number