diff --git a/lisp/vc/vc.el b/lisp/vc/vc.el
index db4fa76abd4..51d0d20ae3e 100644
--- a/lisp/vc/vc.el
+++ b/lisp/vc/vc.el
@@ -1275,6 +1275,8 @@ the revision ID or branch ID."
       (error "Fileset files are missing, so cannot be operated on"))
      ((eq state 'ignored)
       (error "Fileset files are ignored by the version-control system"))
+     ;; Fileset comes from a diff-mode buffer, see
+     ;; 'diff-vc-deduce-fileset', and the buffer is the patch to apply.
      ((eq model 'patch)
       (vc-checkin files backend nil nil nil (buffer-string)))
      ((or (null state) (eq state 'unregistered))