diff -r b5040f7ef2cb -r a9285b04999c lisp/textmodes/fill.el
--- a/lisp/textmodes/fill.el	Sat Mar 27 02:27:15 2021 +0900
+++ b/lisp/textmodes/fill.el	Fri Nov 19 14:00:38 2021 +0900
@@ -708,7 +709,10 @@
     (goto-char from-plus-indent))
 
   (if (not (> to (point)))
-      nil ;; There is no paragraph, only whitespace: exit now.
+      ;; There is no paragraph, only whitespace: exit now.
+      (progn
+        (set-marker to nil)
+        nil)
 
     (or justify (setq justify (current-justification)))
 
@@ -784,6 +788,7 @@
       ;; Leave point after final newline.
       (goto-char to)
       (unless (eobp) (forward-char 1))
+      (set-marker to nil)
       ;; Return the fill-prefix we used
       fill-prefix)))