diff --git a/scripts/upgrade b/scripts/upgrade
index b5b457b..93e457e 100644
--- a/scripts/upgrade
+++ b/scripts/upgrade
@@ -23,7 +23,7 @@ upgrade_type=$(ynh_check_app_version_changed)
 #=================================================
 ynh_script_progression --message="Ensuring downward compatibility..."
 
-if [ -z "$blog_admin" ] ||
+if [ -z "${blog_admin:-}" ] ||
  [ -z "$blog_contentadmin" ] || 
  [ -z "$blog_usage" ] || 
  [ -z "$blog_publish" ] ||