diff --git a/lisp/vc.el b/lisp/vc.el
index 6a0742b8de5..b76fc7de31d 100644
--- a/lisp/vc.el
+++ b/lisp/vc.el
@@ -89,6 +89,10 @@ The value is only computed when needed to avoid an expensive search.")
 (defconst vc-maximum-comment-ring-size 32
   "Maximum number of saved comments in the comment ring.")
 
+;;; This is duplicated in diff.el.
+(defvar diff-switches "-c"
+  "*A string or list of strings specifying switches to be be passed to diff.")
+
 ;;;###autoload
 (defvar vc-checkin-hook nil
   "*List of functions called after a vc-checkin is done.  See `run-hooks'.")