diff --git a/lisp/term/vt100.el b/lisp/term/vt100.el
index 83fcc42d6e1..3092a405fec 100644
--- a/lisp/term/vt100.el
+++ b/lisp/term/vt100.el
@@ -39,7 +39,7 @@
 (load "term/lk201" nil t)
 
 ;;; Controlling the screen width.
-(defconst vt100-wide-mode (= (frame-width) 132)
+(defvar vt100-wide-mode (= (frame-width) 132)
   "t if vt100 is in 132-column mode.")
 
 (defun vt100-wide-mode (&optional arg)