diff --git a/modules/ui/zen/config.el b/modules/ui/zen/config.el
index 6ec42b7dc..c465e6795 100644
--- a/modules/ui/zen/config.el
+++ b/modules/ui/zen/config.el
@@ -10,7 +10,7 @@
   "Pixel size of window dividers when `writeroom-mode' is active.")
 
 
-(defvar +zen--old-window-divider-size window-divider-default-bottom-width)
+(defvar +zen--old-window-divider-size nil)
 
 
 ;;
@@ -31,9 +31,10 @@
     (defun +zen-toggle-large-window-dividers-h ()
       "Make window dividers larger and easier to see."
       (when (bound-and-true-p window-divider-mode)
-        (if writeroom-mode-hook
-            (setq +zen--old-window-divider-size (cons window-divider-default-bottom-width
-                                                      window-divider-default-right-width)
+        (if writeroom-mode
+            (setq +zen--old-window-divider-size
+                  (cons window-divider-default-bottom-width
+                        window-divider-default-right-width)
                   window-divider-default-bottom-width +zen-window-divider-size
                   window-divider-default-right-width +zen-window-divider-size)
           (setq window-divider-default-bottom-width (car +zen--old-window-divider-size)