diff --git a/modules/feature/popup/autoload.el b/modules/feature/popup/autoload.el
index 035d9a83d..5b0e9e386 100644
--- a/modules/feature/popup/autoload.el
+++ b/modules/feature/popup/autoload.el
@@ -416,6 +416,8 @@ prevent the popup(s) from messing up the UI (or vice versa)."
 ;;
 
 (when (version< emacs-version "26")
+  (defvar window-sides-reversed nil)
+
   (defun window--sides-reverse-on-frame-p (frame)
     "Return non-nil when side windows should appear reversed on FRAME.
 This uses some heuristics to guess the user's intentions when the