diff --git a/lisp/diff.el b/lisp/diff.el
index 155942a8582..ea63a2a28ea 100644
--- a/lisp/diff.el
+++ b/lisp/diff.el
@@ -45,7 +45,7 @@
 ;;;###autoload
 (defcustom diff-command "diff"
   "*The command to use to run diff."
-  :type string
+  :type 'string
   :group 'diff)
 
 (defvar diff-regexp-alist