diff --git a/lisp/abbrev.el b/lisp/abbrev.el
index 5111fc59013..7e60541aaf0 100644
--- a/lisp/abbrev.el
+++ b/lisp/abbrev.el
@@ -35,7 +35,7 @@ define global abbrevs instead."
   :group 'abbrev-mode
   :group 'convenience)
 
-(defun abbrev-mode (arg)
+(defun abbrev-mode (&optional arg)
   "Toggle abbrev mode.
 With argument ARG, turn abbrev mode on iff ARG is positive.
 In abbrev mode, inserting an abbreviation causes it to expand